Hybrid Energy Storage Systems in Rail Transport

Abstract

As of now, decarbonization is a central theme. In the European context, 40% of railway lines are operated by diesel trains. Among countries such as Italy, Germany or UK, even values as high as 60% are reached. In order to successfully achieve the 2030 and 2050 targets, in the past it has been considered to electrify all remaining lines. Hybrid trains, however, are an alternative to this. In this chapter, solutions for Hybrid Energy Storage Systems in rail transport will be discussed.

1 Introduction

Environmental factors related to rail transport are Green House Gases (GHG) and local pollutant emissions. GHGs evaluation must be computed with a well-to-wheel approach (WTW), which considers both emissions from the conversion and use of fuel to operate the vehicle, and emissions from fuel production and fuel transport (Choi and Song 2018; da Fonseca-Soares et al. 2024; Kwakwa et al. 2023). The level of Carbon depends on the type of rolling stock supply. To illustrate the advantages of one traction system over another, it is necessary to analyze aspects both from the point of view of the emissions that this traction system entails and the cost of implementing a traction system. Diesel traction has higher CO₂ emissions than electric traction, although on low-traffic lines it can be more economical than electric traction. It must be kept in mind that electric traction has a fixed initial cost, so the construction of an electrified line does not depend on the number of trains running on it. Still, the economic return on construction depends on the useful traffic on the line (Ogunkunbi and Meszaros 2023; Sasse and Trutnevyte 2023). The useful traffic is the number of people carried for passenger trains and tons of goods for freight trains. Therefore, diesel traction is used today for complementary lines with low traffic density with single track, while lines with high traffic density prefer electrified lines. Analyzing more in detail, diesel traction has the energy source on board and has torque at low speeds, which makes it perform better with higher acceleration, but as mentioned earlier, it has higher emissions than electric traction (Praticò and Fedele 2023). The introduction of the Battery Electric Multiple Unit (BEMU) would result not only in reduced emissions but also in the introduction of the electric energy source on the trainset represented by the traction battery. Moreover, also the emissions generated by fuel consumption in the production of electricity for traction should be accounted for (Kapetanović et al. 2024; Klebsch et al. 2018, 2019). For the electric rolling stock, the increase of renewable sources for electricity production must be considered in GHG emissions calculation (see Fig. 1). Since 2017, many railway companies in Europe have purchased electricity from renewable sources, reducing CO₂ emissions per passenger by 15% compared to purchasing electricity directly from national operators. As far as freight transport is concerned, there has been a reduction of around 6%. The European Union announced on 16th December 2020 that 2021 would be the European Year of Railways (IEA, 2020).

Fig. 1

Greenhouse gas emissions from transport

This initiative goes towards the promotion of environmentally friendly public transport for both companies and citizens and is part of the goal of achieving climate neutrality by 2050 sought with the Green Deal. Transport accounts for 25% of greenhouse gas emissions in Europe. However, rail only accounts for 0.4% of these emissions and is the only sector that has drastically reduced emissions since 1990. Only 11% of passengers and 7% of goods travel by rail. In addition to this, the European Union wants to increase these percentages to reduce greenhouse gas emissions, especially from the transport of goods by land and sea. During the Covid pandemic, the railway was a means of transport for essential goods, including medicines and basic services. This sector was also severely affected by a decrease in passenger numbers due to health restrictions imposed by European states in response to the pandemic. A supplementary package that created a united railway area without institutional barriers in favor of economic growth has also been designed (European Parliament 2022).

As it was reported earlier with the Green Deal, Europe has set a target to reduce greenhouse gas emissions by 50% by 2030 and 90% by 2050 compared to 1990. In Europe, most railway lines are electrified, although there is a 20% use of diesel rolling stock. On lines with low passenger density, the electrification process is not economically sustainable. Alternatives to diesel propulsion are being investigated, such as the introduction of hydrogen and the use of battery-powered trains (BEMU) that can reduce the level of greenhouse gases (G. and I. T. Directorate-General 2021).

Over the decades, the technology of a Diesel Multiple Unit (DMU) has improved considerably in terms of both emission and noise reduction and energy absorption during operation. Many DMUs feature a battery to ensure hybrid operation (Praticò and Fedele 2023). However, European Union has decided to stop the production of diesel vehicles; this decision makes clear that the objective of decarbonization in rail transport is to be achieved by moving in a different direction, namely through the introduction of new technologies (Ansa 2022).

In the classification of railway lines three classes are distinguished:

• Fully electrified lines which are subdivided into regional transport lines and high-speed lines, where run Electrical Multiple Unit (EMU). The rolling stock operation consists of the absorption of electrical power through the catenary by means of a pantograph. Electrified lines in the European Union have different traction systems that differ in terms of catenary voltage. For historical reasons too, most of the European network has an electrical voltage of 15 kV AC single-phase with a frequency of 16.7 Hz or 25 kV AC single-phase with a frequency of 50 Hz for ordinary regional rail service. In Italy, there is a 3 kV DC power supply for regional rail transport (Arboleya et al. 2020). Furthermore, the Technical Specifications for Interoperability (TSI) for Energy subsystem establish in Sects. 7.2.1 and 7.2.2 that the railway lines of the various countries must be interoperable, although it is left up to each country to arbitrarily choose the traction system (Commission Regulation 2014).

• Non-electrified lines that do not have any power supply system are travelled by DMU and by diesel-electric rolling stock.

• Mixed or hybrid lines are those railway lines with partial electrification. Partial electrification is due to the presence of electrified line sections and stations since these are shared with other lines that are fully electrified. Such lines are run by DMU or Battery Electrical Multiple Unit (BEMU).

The railway represents a transport sector that is widely used to connect isolated areas of Europe and thus represents a link between internal and cross-border European regions. In Italy, this situation is not the same case as European contest because many areas of Italy are not connected by any railway lines.

In the Italian railway sector, a distinction must be made between basic lines and complementary lines. Fundamental lines are those lines with a high level of traffic and an electrified infrastructure connecting the main cities, while complementary lines are single-track lines with medium to low traffic and a non-electrified infrastructure. From an infrastructural point of view, mixed lines are complementary lines with a low level of electrification compared to the entire length of the line. The three lines here investigated, which run from the city of Pavia, are medium-low traffic lines and have a medium-low electrification level concerning the length of the line. Diesel traction is the solution used on these sections because any electrification of the line would entail an unsustainable cost with respect to the traffic on these lines. Traditional diesel traction is flanked by diesel-electric traction to reduce the environmental impact of rolling stock. BEMU is an economical and environmentally sustainable solution for these lines. BEMUs are rolling stock with a traction battery that can be used on sections of line where there is no catenary, while where there is a catenary, the rolling stock is powered using a pantograph. The pantograph allows the battery to be recharged both while running and when the rolling stock is at a standstill at the station. BEMU is used in some European countries such as Germany, France, Austria, Netherlands, England, and Norway (CHAMARET 2019; IEEE Industrial Electronics Society 2019; Thorne et al. 2019; Heckele et al. 2022), while it is not yet used in Italy.

2 Main Trends in Rail Transport Sector

Mobility as a resource for environment and community: public transit, specifically the one on two wheels and rail, needs to be decarbonized in order not to further impact the Planet’s health and, at the same time, allow to move around in an efficient and sustainable way.

• On a global scale, rail industry is undergoing a transformation, with hybrid trains emerging as the next key technology, towards a sustainable and efficient rail transport. Such trains, combining traditional diesel motors with electric traction systems, offer a practicable solution in reducing GHG emissions and fuel consumption. In the following, the five main trends that shape hybrid train market sales are presented:

• Growing emphasis on environmental sustainability: Environmental sustainability is the leading aspect of hybrid train markets. Governments and regulatory bodies all over the world are setting stricter and stricter targets for emissions, to fight against climate change and reduce atmospheric pollution. Hybrid trains are seen as a crucial part of this strategy since they can significantly reduce carbon emissions concerning conventional diesel trains. These trains are able to circulate on non-electrified lines, which compose the largest share of global railway lines, using energy coming from batteries in urban areas to reduce to the maximum extent possible local pollution. The push towards more ecological transport solutions is stimulating the demand for hybrid trains, with producers focusing on developing more efficient and more ecological models every year.

• Technological progress in batteries and energy storage systems: one of the most relevant tendencies in the hybrid train market is the rapid evolution of batteries’ technology and energy storage systems. Innovations, such as the ones in the field of Lithium-Ion and solid-state batteries, as well as other storage solutions, are improving the performances, autonomy and efficiency of hybrid trains. These progresses allow for longer working periods in electrical-only mode, reducing the dependence on diesel motors. In addition to this, regenerative braking systems, able to capture and store energy during braking, are becoming more and more diffused. These technologies not only play in favor of making hybrid trains more ecological and sustainable but also reduce operative costs, acting on fuel consumption and maintenance requirements.

• Government incentives and funding programs: Governments all over the world are introducing incentives and funding programs to promote hybrid trains’ adoption. These initiatives include grants, subsidies, and tax relief projects for railway operators and producers that invest in hybrid technology. As an example, in Europe, the European Green Deal is aimed at making railway transport more sustainable, distributing significant funding for the development and diffusion of hybrid trains. Parallel to this, in Asia and North America, governments are prioritizing projects of railway infrastructures that contemplate hybrid technologies. These incentives are accelerating the market’s growth, making hybrid trains more appealing and financially feasible for operators.

• Expansion of hybrid trains’ applications: The possible applications of hybrid trains are growing beyond passenger service, including the transport of goods and high-speed service. Freight transport operators are adopting an ever-increasing number of hybrid locomotives in order to keep up with the targets set for emissions reductions and raising fuel efficiency. In addition to this, high speed hybrid trains able to guarantee faster long-haul routes, while being cleaner and more efficient are currently being developed. This diversification is enlarging the hybrid trains’ market, because operators, in various fields, recognize the advantages related to lower emissions, lower operating costs, and better performances. The versatility of hybrid trains makes them suitable for a wide range of rail services, further pushing the possibilities of adoption.

• Integration with intelligent and connected technologies: Hybrid trains’ market is being revolutionized by the integration of intelligent and connected technologies. Advanced monitoring systems, IoT devices and data analysis are being used towards optimizing rail operations, increasing energetic efficiency and bettering user experience. As an example, predictive maintenance technologies allow operators to monitor working conditions of hybrid trains in real time, avoiding mechanical faults and reducing the downtimes. Furthermore, intelligent energy management systems allow for a more efficient battery recharge and refueling. These technological progresses are making hybrid trains more reliable, convenient and easy to use, contributing to their ever-increasing popularity.

3 Example Technologies to be Adopted

3.1 Battery Electric Multiple Unit (BEMU)

An important goal for the freight and passenger transport sector is the reduction of CO₂ emissions into the air. In Europe, transport accounts for 25% of greenhouse gas emissions and is the main cause of urban pollution gases (G. and I. T. Directorate-General 2021).

These emissions must also be reduced to ensure a slowdown in climate change that has been affecting the world in recent years. This climate change is due to the greenhouse effect caused by the CO₂ emissions mentioned above.

Europe's response to this problem is the introduction of increasingly low-emission transport. Rail transport is a sector with very low carbon emissions for both passenger and freight transport. However, diesel-powered rolling stock is currently used in this sector for low and medium-traffic density routes.

Through the Green Deal signed by the European Commission, the European Union has set the goal of reducing net greenhouse gas emissions by at least 55% by 2030 compared to 1990 levels, combined to have no more diesel vehicles on the market by 2050, achieve carbon-free transport (European Commission 2022). In the railway sector, substitutes for diesel traction are being considered with the aim of reducing both environmental and noise pollution. Possible solutions are rolling stock with batteries, hydrogen fuel cells, and other low-carbon energy sources. In diesel rolling stock, hybrid rolling stock is considered. Such rolling stock has a diesel-electrified traction that reduces CO₂ emissions into the air (Praticò and Fedele 2023). Complete electrification of the lines is not applied on these railway sections because the cost of electrification is high compared to the useful passenger traffic, and this investment would not be recovered.

In recent decades, special attention has been paid to vehicles with an onboard energy storage system for use in railway sections where there is no catenary. Such OESS (Onboard Energy Storage System) represents a very efficient electrical traction with the use of a reasonable amount of regenerative braking. Such vehicles represent a solution as they minimize costs by reducing maintenance and installation of electrical infrastructure requirements.

One problem with the use of these rolling stock materials is the materials used to manufacture the batteries. Rail represents the lowest carbon-emitting transport sector today and the most efficient, although a 9% share of total passenger and freight transport railways account for less than 2% well-to-wheel GHG, and it is about 3% of total final energy use (IEA 2022). Moreover, the low energy demand per passenger per kilometer is because there are lower losses caused by friction and drag. The higher energy efficiency of electric motors compared to diesel engines is due to regenerative braking and the high storage capacity; however, in the account of GHG emissions related to electrification in railways, emissions of fuels that are used to produce the electric energy are considered. In electricity production, renewable sources are also considered. In Europe, approximately 40% of electricity is produced with minimal coal, with an average of 20% produced directly from renewable sources (IEA 2022).

All over the world, solutions are being developed to reduce the environmental impact of railway companies’ rolling stock by increasing their production and power plants to promote energy demand interaction. Production of energy from renewable sources has increased, including nuclear energy, and this production contributes to supply railway traction. In the global railway world, the mix of renewables and nuclear energy accounted for 13.5% as of 2015, and it has doubled since 1995.

BEMU technology

Bimodal battery trains have replaced diesel-powered rolling stock in complementary hybrid lines in some European countries, including Germany, England, France, Austria, and Poland. Worldwide, in some states such as Japan and even Russia, bimodal battery trains are used, while in North and South America, diesel traction is still widely used (IEA 2022).

BEMU is a train with battery traction where there are no electrified sections on a line, while there is a catenary. The train through the pantograph absorbs power to recharge the battery. The problem of the current absorption limit that occurs when the train is stationary in the station to recharge does not appear in the case of recharging in a running train. While running, the train with the pantograph raised in the electrified sections can absorb currents that are in the order of magnitude larger than the 200 A imposed by the TSI Energy standards (RFI 2022). The use of a BEMU over a diesel drive also has the beneficial effect of reducing particulate matter and nitrogen oxides NO_x (NASA 2022).

Returning to the discussion of the different railway infrastructures for regional lines in European states, the presence of a higher voltage value for the same absorbed current results in greater power absorbed by the rolling stock for charging the catenary batteries. Higher power results in a shorter charging time for the trainset battery so that the use of the BEMU in the railway company commercial services is feasible.

In Italy, as there is a lower voltage value, charging times are longer and are not compatible with the railway companies’ dwell times, thus limiting the use of such trainsets for passenger transport on mixed lines. The lines, where BEMU can operate, are lines with a low to medium useful traffic index and have electrified sections at least in the line head stations for charging the rolling stock.

BEMU has the traction system on board, so on the lines where it is applied, it does not need any major infrastructure intervention. On railway lines with a low average useful traffic index, BEMU represents an opportunity and a solution that minimizes infrastructure maintenance costs. However, the use of on-board batteries has some limits because battery trains are used on lines that are 60 to 100 km long, while this parameter also varies depending on the rail line and schedule characteristics (Streuling et al. 2020). Another limitation of the battery-powered train is that on lines where there is a very steep gradient, the status charge of the battery drops more quickly. The batteries that are mainly used as traction batteries for bi-modal battery trains are nickel metal hydride (Ni-MH) and lithium-ion (Li-ion) batteries. Ni-MH batteries have a very high robustness with low maintenance, although they have a low efficiency and a high self-discharge rate. However, the batteries that are being researched the most are lithium batteries, which have higher energy densities with higher efficiency and a lower self-discharge rate. Li-ion batteries are often considered to be homogeneous groups and so the batteries with high energy density include nickel-metal cobalt and lithium manganese oxide, but they have short lifespan (Ghaviha et al. 2017). BEMU trains are similar in size to EMU trains. The only difference is the greater weight of the rolling stock due to the presence of the traction battery. Through roof-mounted pantographs, BEMU operates like a normal EMU on an electrified route. Logically, the battery is used for the non-electrified sections while the pantograph is used as a charging system for the batteries both when the train is moving and when it is at a standstill at the terminus station.

The catenary is a very fast charging system for the batteries as it can provide the battery with power in the order of MW that can recharge the traction batteries quickly (Heckele et al. 2022). Nowadays, the performance of a BEMU depends on the distance of the railway line without a catenary and the capacity of the battery. One parameter that must be considered is the degradation of battery capacity due to load utilization. Other factors that determine a battery utilization limit are stringent running conditions and the climatic peculiarities of the line. These factors have an impact on the utilization range of the traction battery. Manufacturers still lack the capabilities and data to determine the long-term impact of these factors on battery performance. This leads manufacturers to force the use of traction batteries for specific lines where simulations of driving profiles have been made (Heckele et al. 2022).

The useful capacity of the battery does not correspond to the nominal capacity of the battery specified in the battery datasheet. Normally, an attempt is made to have a state of charge (SoC) of the battery in a range between 20 and 80% to preserve the capacity of the battery and thus extend its service life as much as possible, so the capacity is also designed to work in a state of charge (SoC) between 40 and 80%. In the battery design, the cases of failures and extreme weather conditions are also considered to establish a buffer that for a defined short time can guarantee more power without damage to the battery. Such situations are considered to avoid the case where the BEMU stops due to battery depletion.

One aspect that greatly increases the efficiency of the BEMU is regenerative braking because if there is no other train in the vicinity that can absorb this energy to accelerate, the train uses this energy to recharge the traction battery (Heckele et al. 2022).

After presenting all the main advantages of using a BEMU train, the negative aspects of adopting such a means of transport are also discussed. One disadvantage is that the BEMU is 10% heavier than a fully electric train due to the weight of the traction batteries.

Another disadvantage is the construction of charging points for the batteries and the organization of charging times throughout the day. These times must be compatible with the dwell times required by the railway companies. The addition of recharging times for battery trains can lead to problems with the smooth running of the train service provided by the railway undertakings and require changes to the infrastructure, as such high recharging capacities require ad hoc recharging solutions for possible stations (Heckele et al. 2022).

The main problem for eventual use is the charging time of battery trains in stations. Energy TSI stipulates that a standstill train can draw a maximum current of 200 A from the catenary. This current limit is set because the elastic catenary, being under tension, heats up at the point of contact between the pantograph creep and the contact wire and the contact wire can break, causing damage to the infrastructure (European Commission 2014).

This maximum current limit is one of the most critical points for the use of BEMU in some European countries. In Europe, the infrastructure for regional railway lines is supplied with a 15 kV AC single-phase power supply, whereas in Italy these lines are supplied with a 3 kV DC power supply. This difference in power supply systems for the traction of electric or battery-powered rolling stock results in a substantial difference in the recharging time of BEMU at the end of the line (Arboleya et al. 2020). The difference from a rolling stock with diesel traction is represented by the fact that BEMU has a traction system on board the train, however, BEMU requires a charging infrastructure for the battery, which entails a modification to the existing railway infrastructure.

Charging and filling infrastructure

Battery trains can use the catenary in electrified sections and the battery in non-electrified sections as the traction system. This means that the battery train in electrified sections uses the pantograph, which, when placed in contact with the catenary, draws electric power. In this operating regime, BEMU behaves like an EMU train; in addition to that, BEMU can use the catenary contact line to recharge the battery when it has a low state of charge, and the rolling stock is in motion. Based on the rolling stock running in Europe and Japan, the various charging systems for each BEMU are listed. In Europe AC railway power supply system’s generally operate at 15 kV at 16, 7 Hz or 25 kV at 50 Hz. These voltage values are standardized by EN 50,163, and to ensure maximum interoperability between the various European railway systems, the technical specifications for energy interoperability consider these two standards.

EN 50,163 stipulates the use of a frequency of 50 Hz for recharging infrastructure in the case of a nominal voltage of 25 kV AC. However, battery-powered trains in states with a power supply of 15 kV AC with a frequency of 16.7 Hz must recharge with a frequency of 50 Hz. To recharge a multisystem vehicle for both AC traction systems, a single transformer with two voltage taps on the primary side can be used to change the number of transformer turns to change the transformer turn ratio.

Unlike vehicles that have a single traction system, multisystem vehicles need an additional switch in the primary side of the transformer, and components that are subject to a voltage of 25 kV must be sized for a higher voltage value. Consequently, standard EN 501 63 indicates a maximum voltage value of 29 kV for the nominal value of 25 kV.

A reason why a higher voltage is used at the same frequency, which in this case is the frequency of 50 Hz, is that the inductive interference of voltages is proportional to the frequency of the voltage, so a higher voltage value compensates these effects.

An alternative solution to this problem may be to reduce the distance between substations (Dschung and Ludolf 2021). In isolated lines, this problem is irrelevant as the reactance of the line is negligible.

In Germany, Talent 3 can recharge through the 15 kV overhead line at a frequency of 16.7 Hz or use regenerative braking like Electrostar and D-Train in the UK (Thorne et al. 2019). In Japan, the EV-E301 V uses a DC/DC converter to lower the voltage from the 1500 V DC of the overhead line to the 630 V of the battery voltage. In non-electrified lowered pantograph sections, BEMU can recharge its battery pack through regenerative braking (Thorne et al. 2019).

3.2 Hydrogen Trains

An alternative, clean and with great potential, that is currently acting towards reducing the emission of large amounts of carbon dioxide in the atmosphere is Hydrogen.

In Europe, hydrogen train experimentation has already begun, as a new frontier of sustainable mobility. Hydrogen trains represent the ecological evolution of rail transport modes, that in these years have traditionally been powered by fossil fuels. It is a new transport technology that allows carriages to move using entirely clean energy, fully respecting the environment. Green hydrogen is a zero emission energy vector, renewable, and very efficient: in the specific, it allows for high performances thanks to the capacity to carry a large amount of energy per kg of fuel. According to estimates, the employment of a hydrogen train can avoid over 4.400 tons of CO₂ emissions in a single year. In this light, it is possible to understand why they represent a green solution to pollution and global warming.

In terms of structure, on top of the carriage a fuel cell is installed, acting as the nucleus of the whole system, and allowing to mix the hydrogen located in the tanks with oxygen naturally present in the environment. The only emissions that are produced by this green train are vapor and condensate water, two of the components generated by the meeting of hydrogen with oxygen in the fuel cell: a 100% green and eco-friendly mobility.

A completely new scenario is being opened by hydrogen trains in transport systems. This paradigm shift is registering as a first improvement the cutting of a notable amount of GHG emissions, around 40%. Moreover, if on top of green hydrogen also grey hydrogen is used (the nomenclature derives from the employment of fossil sources for hydrogen production, typically natural gas), an overall lessening of pollution concerning the oil engines currently operating is to be expected anyway.

In a not-so-distant future, in which an increase of hydrogen production is estimated, the cost of the green energy vector will be more competitive with respect to diesel.

Another benefit is in terms of the refuelling process. In the future it will become a fast operation, in the order of minutes, allowing for a drastic reduction of stopping times for trains. This, combined with the trains being able to run for 18 consecutive hours between refuelling stops, will provide an unprecedented level of service.

In some European countries, such as Germany, hydrogen passenger trains are already fully operational and regularly used by travellers. In UK and France, some proposals have been brought forward in order to fully substitute diesel trains with hydrogen ones, in the next twenty years, in those lines that are difficult to electrify. In Italy, as it’s possible to read in the Strategical National Guidelines for Hydrogen, as much as half of non-electrifiable national lines could be converted to hydrogen before 2030. A more sustainable and not-too distant-tomorrow.

In Table 1 it is possible to see a hydrogen SWOT analysis, identifying points of strength, weakness, opportunities and threats for this technology.

Table 1 Hydrogen SWOT analysis

4 Guideline for the Electrification of Rail Infrastructure

Climate change is the challenge of our time that will determine our future. Climate change is accelerated by the presence of greenhouse gases in the atmosphere, of which carbon dioxide (CO₂) is the most significant. Transport is one of the sectors with the greatest responsibility for CO₂ emissions (IEA2023a, 2023b). In transportation, the road infrastructure segment needs major decarbonization measures, but it is not the only one. Non-electrified railway lines represent a major sustainability challenge, and currently, about 33% of lines in Italy use diesel traction, contributing significantly to greenhouse gas emissions (García-Olivares et al. 2020). Electrification through the implementation of new catenary infrastructure is not always easy to implement due to geographical and structural constraints, such as the presence of tunnels and bridges. At the same time, the implementation of new catenary electrification systems may not be economically viable for short line lengths or short sections where catenary electrification does not exist. Catenary electrification is the traditional method of reducing emissions in rail transport. It offers high energy efficiency but involves high infrastructure costs (around €1 million/km) and it is more suitable for high-frequency lines (Kilsby et al. 2017). To address these requirements, this chapter provides guidance on how to make such lines more sustainable by analyzing two technological solutions:

• Battery-powered trains;

• Hydrogen fuel cell trains.

4.1 Technical Solutions for Railway Electrification

Battery Trains

Battery-powered trains are a promising and adopted solution for short to medium-distance non-electrified railways. The technology is based on the use of lithium batteries, which are known for their high energy density and efficiency. Battery trains can also operate in bimodal mode, using the energy stored in the batteries and using electrified sections of track. This technology involves the use of lithium batteries, which are already used in vehicles because of their energy efficiency and the possibility of recharging them at dedicated stations or sections via the overhead contact line. Table 2 summarizes the advantages and limitations of battery trains (Pugi et al. 2024; Abiko 2013).

Table 2 Advantages and limitations of battery trains

Many applications require the use of batteries sized for the type of service to be performed. For correct sizing, it is advisable to first simulate the total energy consumption to be combined with the required range and operating conditions. Battery trains are a viable solution for regional lines of medium length and moderate frequency. However, a detailed planning of the charging phase should be defined and adapted to the service to be provided. The use of regenerative braking in this scenario is a very useful tool to increase the available energy of the vehicles. Finally, the rapid development of traction batteries in terms of energy efficiency could provide a further boost to the use of these vehicles, enabling them to travel ever longer distances.

Hydrogen Trains

Hydrogen cell trains are an innovative solution for decarbonizing non-electrified railways, especially in regions with long distances between stations or complex geographical conditions. This technology uses hydrogen as the primary fuel for the cells to generate electricity through electrochemical oxidation, producing water vapor and avoiding CO₂ emissions. Although fuel cells have significantly lower efficiency than lithium batteries, their energy efficiency is much higher than that of an internal combustion engine. Today, fuel cells with efficiencies over 60% are being investigated for transport applications. Table 3 summarizes the advantages and limitations of hydrogen trains (Ding and Wu 2024; Nqodi et al. 2023).

Table 3 Advantages and limitations of hydrogen trains

The most widely used fuel cell technology for traction is PEM (polymer electrolytic membrane). Hydrogen trains are an optimal solution for long non-electrified lines due to their autonomy and ability to operate without extensive electrical infrastructure. However, the initial cost of refuelling infrastructure and the production of green hydrogen remain the main barriers to large-scale deployment. The choice of this technology depends heavily on local conditions and sustainability priorities.

4.2 Technical Analysis Implementation

Battery Trains

As explained above, in order to implement a service based on battery trains, it is necessary to know the energy demand on the line through a simulation. To do this, it is necessary to select a type of rolling stock and build a consumption model (Colombo et al. 2023). The next step is to start the battery sizing process as it follows. The limitations introduced for the battery sizing were described in (1, 2):

$${\text{P}}_{{{\text{disch}}}} = {\text{P}}_{{{\text{train}}}} + {\text{P}}_{{{\text{aux}}}} + {\text{P}}_{{{\text{add}}}}$$

(1)

$$E_{tot} = \eta_{disch} \cdot \frac{{P_{disch} }}{{C_{disch} }}$$

(2)

where:

• $P_{disch}$ is the discharging power, the useful power supplied by the battery to ensure both traction and auxiliary services.

• $P_{train}$ is traction power;

• $P_{aux}$ is the power needed for the auxiliaries

• $P_{add}$ is a set condition to be sure to cover all the requirements.

• $E_{tot}$ is the total energy provided by the battery;

• $\eta_{disch}$ is the battery efficiency;

• $C_{disch}$ is the discharging rate of the battery.

For the lithium-ions battery it is possible to assume a complete charge/discharge cycle to 80%. C-rate, instead, identifies the discharge rate of the battery linked to its capacity. This highlights the discharging current at which the battery begins to discharge over its nominal capacity. Having to size the battery satisfying both the power and the energy requirements to complete the journey, it is necessary to have an estimation of the energy consumed ($E_{cons}$) with the addition of a margin ($E_{add}$) for non-regular operation or stress conditions so that the battery does not discharge completely (3).

$$E_{cons} = E_{tot} + E_{add}$$

(3)

Therefore, the maximum one needs to be selected between the two obtained values and this value must be used to estimate the total necessary capacity of the battery ($C_{tot}$) (4).

$$C_{tot} = \frac{{{\text{max}}\left( {E_{cons}, E_{tot} } \right)}}{{V_{batt} }}$$

(4)

The battery installed on-board reaches the desired voltage by connecting a number of cells in series ($N_{s}$), each of which increases its overall potential by its own voltage. The parallel connection instead increases the total capacity instead ($N_{p}$). It is important to use the same type of cell with the same voltage ($V_{cell}$) and capacity ($C$), since modules made from cells with different voltages, capacities and dimensions could cause imbalances (5, 6).

$$N_{s} = \frac{{V_{batt} }}{{V_{cell} }}$$

(5)

$$N_{p} = \frac{{C_{tot} }}{C}$$

(6)

The series–parallel configuration allows to reach the desired voltage and current values starting from a certain number of standard cells. According to the total number of cells and the number of cells connected in parallel, it is useful to compute more accurately the new value of energy needed to satisfy the highlighted specifications $E_{tot\_new}$ (7, 8) and the maximum absorbed current by the battery $I_{max}$ (9).

$$C_{tot\_new} = C \cdot N_{p}$$

(7)

$$E_{tot\_new} = C_{tot\_new} \cdot V_{batt}$$

(8)

$$I_{max} = C \cdot C_{rate} \cdot N_{p}$$

(9)

The maximum absorbed current value is necessary to compute the battery charging power $P_{ch}$ (10), so as to have an estimation of the recharging time necessary $t_{ch}$ (11), taking into account also the energy consumed for the journey.

$$P_{ch} = I_{max} \cdot V_{batt}$$

(10)

$$t_{ch} = \frac{{E_{cons} }}{{P_{ch} }}$$

(11)

The charging time is an element that must be considered with the location and management of the charging stations since the recovable share of regenerative braking energy is not enough for recharging the battery. Furthermore, the good performance of the battery also depends on this parameter. The required performance directly affects the system’s energy consumption. Therefore, knowing that the necessary energy depends on load, structural characteristics of the lines, speed profile and so on, an increase in the total mass of the train due to the addition of the battery on-board could have an impact on the overall system (12). This highlights the weight of the battery after the sizing process, combined with the specific energy of the storage system ($e$).

$$m_{batt} = \frac{{E_{tot\_new} }}{e}$$

(12)

Figure 2 highlights the process to be followed for the battery train implementation.

Fig. 2

Battery train implementation process

Hydrogen Trains

Similarly, in the case of hydrogen electrification, a test drive is required to determine consumption and, again, a suitable vehicle should be selected based on the characteristics of the route. If a vehicle with these characteristics does not already exist, it is advisable to refer to another vehicle with similar characteristics. Once the profile for energy and power is simulated, it is possible to provide the battery and fuel cell sizing. The fuel cell sizing is developed depending on the average power required to operate the line. The required energy will be provided by the fuel cell, which will supply the battery or the traction motors alternatively (Iskandar and Maheri 2022). Thus, the hydrogen on board will be enough to supply all the load, when the battery will satisfy the peak demand during traction. A coherent evaluation of the energy implies the use of (13).

$$E_{tot} = \frac{{E_{el1} }}{{\eta_{FC} }} + \frac{{\left( {\frac{{E_{el2} }}{{\eta_{FC} }}} \right)}}{{\eta_{batt} }}$$

(13)

where:

• $E_{tot}$ is the total energy required for the path expressed in kWh;

• $E_{el1}$ is the energy consumption in case of direct flux fuel cell-motors in kWh;

• $E_{el2}$ is the energy consumption in case of fuel cell supplying both battery and motors in kWh;

• $\eta_{FC}$ is the efficiency of fuel cell assumed constant and equal to 0.6;

• $\eta_{batt}$ is the efficiency of battery assumed equal to 0.9 and constant.

Then, considering now a hydrogen energy density equal to 5.6 MJ/l (compressed hydrogen at 700 bar) [40], it is possible to obtain the volume of required H₂ ($V_{{H_{2} }}$) as a function of the total energy ($E_{tot}$) and the energy density ($u$) (14).

$$V_{{H_{2} }} = \frac{{E_{tot} }}{{u}}$$

(14)

Since hydrogen tanks can contain up to 0.039 kg/l at 25 °C and 700 bar, it is possible to estimate the H₂ mass ($m_{{H_{2} }}$), knowing the tank capacity ($C_{tank}$) (15).

$$m_{{H_{2} }} = V_{{H_{2} }} \cdot C_{tank}$$

(15)

However, it must be considered that the train is not operating while carrying the H₂ mass. The weights of the on-board devices, the cooling systems, the air management and the whole tank, must be considered, while considering the weight of the fuel cell. Then, to size the lithium batteries to side to the fuel cell, the maximum energy and power need to be evaluated. It is necessary to size in energy and power and after adopting the major value between the two (16, 17).

$$m_{Li\_p} = \frac{{\Delta P_{max} \cdot 1000}}{{p \cdot \eta_{batt} }}$$

(16)

$$m_{Li\_e} = \frac{{\Delta E_{max} \cdot 1000}}{{u \cdot \eta_{batt} }}$$

(17)

With $m$ indicating the mass for the two different types of sizing. Specifically $\Delta P_{max}$ measured in kW is the difference between the providable power from tank and required power and $\Delta E_{max}$ in Wh represent the difference between the providable energy from tank and required energy. $p$ represents power density [W/kg] and $u$ energy density [Wh/kg]. Figure 3 reports the overall sizing process for hydrogen/battery hybrid train.

Fig. 3

Hydrogen train implementation process

Catenary-less Electrification Outcomes

After the analysis of the methodologies of catenary-less electrification, two relevant factors prevent the adoption of a new construction of electrified networks. The type of the lines is an impacting factor, secondary branches in regional network are non-interested by heavy traffic, meaning the electrification is not profitable. Typically, electrified lines have a considerable frequency, in fact usually capacious trains are employed. Thus, since relevant economic investments are needed to transform the paths, it would be safer saving efforts for higher density routes. Battery-traction trains through the preliminary sizing, under favorable assumptions, are solutions that could be effectively adopted, since they satisfy primary constraints. Starting from power and energy as requirements, a light train could be transformed to a battery-traction train with the state-of-art technology. In this sense, it was a confirmation that this is an enough-mature alternative for non-electrified tracks. On the other hand, longer pathways could exploit the features of hydrogen-trains. Again, from power and energy demand it was proved a preliminary and non-optimized sizing of a light train with fuel cell stack and battery pack. In this case, the adoption of equipment for hydrogen provision could result simpler and suitable for the line. In addition, other possibilities of optimization, also from a technological point of view, can be evaluated but this brief report can be intended as an initial backbone for further studies.

Operating Cost Estimation Process

Finally, it is worth highlighting the costs that would be incurred with each of the proposed solutions compared to the baseline scenario, which uses diesel for traction. Figure 4 proposes the model to be considered when assessing the costs of (a) diesel, (b) decarbonised with battery, (c) decarbonised with hydrogen.

Fig. 4

Operating costs estimation process for: a diesel train, b battery train, c hydrogen train

As Fig. 4 shows, the processes for estimating operating costs are similar. The process shown in Fig. 4c is also similar to that shown in Fig. 4a and b, except that in the case of hydrogen trains, operating emissions costs are not considered. Instead, these costs need to be considered in the electrolysis hydrogen production phase, depending on the sources feeding the electrolyser.

5 Technical–economic and Sustainability Analysis

In order to evaluate possible investments and in general the uptake of such innovative technologies, Multi-Criteria Decision Analysis (MCDA) is considered. This method is a structured process for assessing options with conflicting criteria and choosing the best result. MCDA is analogous to a cost–benefit analysis but evaluates numerous criteria, apart from just cost. MCDA has operations in several fields, including business, government and everyday life. For illustration, we can use MCDA to decide between vendors or to select equipment that meets all the institution/industry requirements. The steps, needed to perform a Multi-Criteria Decision Analysis effectively, consist in:

• 1. First, by defining the objective: To achieve an ideal mode of transport for the people with which considering the contextual factors for the analysis. In our case, it is (Railways) for Public Transportation.

• 2. Defining the criteria: By developing the criteria to represent the norms for different options. This relates to the measures we assume are important in determining the most valuable choice. We used the parameters such as price (or) cost for transport usage, time taken from origin to destination, distance covered, reliability of the transport and emission control.

• 3. List of choices (or) alternatives: This step is to find choices that meet our criteria to a certain degree. For illustration, we used the criteria to offer affordable and durable characteristics and a list of top three options namely, trains (electrified), buses and self-driven cars.

• 4. Determine the performance values: Each criterion is likely to have its own performance value or the measure to use for ranking it in comparison to other criteria. Price, time and distance use a numeric performance value. Further private criteria such as Reliability and Emission control use different measures, similar to ‘high (or) excellent’, ‘above average,’ ‘average,’ ‘below average’ and ‘low (or) poor.’

• 5. Rate the choices: Rating our choices involves determining how each option compares to identified criteria. A criterion similar to price is a non-beneficial criterion, meaning that a lower value is preferable. For illustration, we probably prefer a mode of transport offering lower prices (in terms of cost only). In our standings, the lowest price would have the higher rank. In comparison, criteria similar to distance cover, time taken, and reliability are beneficial criteria, meaning that advanced values are better and directly proportional to the rankings.

• 6. Normalization of the performance values: Normalization refers to the act of adapting the values so that they operate on a common scale. To do this, we can perform mathematical operations to convert the values. For non-beneficial criteria, divide the smallest value by the performance value. For beneficial criteria, you first convert your values using a conversion scale. The smallest rank would correspond to the numeric one and the highest would correspond to five. After converting the performance values, divide each by the maximum value to homogenize the scale.

• 7. Substitution of Weights: Multiply each by the weights assigned to the corresponding criterion, represented as a numerical value. The sum of the weightage must be equal to 100%. In our case, we assumed weights equally for all the parameters to obtain the Weighted Normalized Decision Matrix.

• 8. Calculating the performance scores: Eventually, after calculation with equal weights for each option, the performance scores (performance factor assumed: (0) Lowest, (1) Highest) are obtained and compared for the investigated alternative options. The option with the highest score would be with the most value, according to the criteria.

6 Conclusion

With the new European directives and the net-zero emissions target set for 2050, the electrification of transport has become an important research topic. In recent years, several studies have been published proposing decarbonisation methods for rail transport that rely on the use of diesel fuel. The most promising technologies are battery trains and hydrogen trains. This chapter analyses both technologies, highlighting their advantages, disadvantages and deployment situations. Moreover, it proposes a methodology for the electrification of railway lines using these technologies. Finally, methods for estimating operating costs were proposed, highlighting the dependence on fuel and energy costs. In conclusion, battery and hydrogen powered trains will be the optimal solution for railway lines that are difficult to electrify, due to their flexibility, energy efficiency and emission reduction, as the technologies progress.

Hybrid trains’ market is heading towards a significant growth, guided by the ever-increasing attention to sustainability, technological progress, government incentives, expansion of applications and integration of intelligent systems. While the global railway industry keeps evolving, hybrid trains offer a valuable solution for a cleaner, more efficient and more versatile rail transport. These tendencies not only emphasize the potential of hybrid trains, but also highlight the innovative steps towards a greener and more connected future in rail transport.