What are the low-carbon alternatives to diesel trains (and how much will it cost)?

You have probably already travelled on board a « Régiolis », train, as there are more and more of them running in France. Their main advantage is their versatility. In their standard configuration, these trains can run both on catenary lines, in electric mode, and on non-electric lines, in diesel mode.

The Régiolis family also includes several other versions, already in service or under development: an electric version capable of operating at different voltages; a biofuel version, tested in 2021; a hybrid electric-diesel-battery version, which has been in commercial service since 2023; and a dual-mode electric-hydrogen version, scheduled to enter commercial service in 2026.

This means that Régiolis trains can operate across the entire rail network, whether electric or otherwise, avoiding the use of diesel locomotives under catenary lines for routes that are only partially electric. For example, prior to 2014, only X TER diesel trains were used on the Paris-Granville line. Nowadays, with the arrival of Régiolis trains, electric power is used from Montparnasse to Dreux, then the trains continue the journey to Normandy by drawing on their fuel tanks.

In 2023, global carbon dioxide emissions, which are the main contributor to climate change, reached 37.4 billion tonnes. Almost a quarter of this can be attributed to the transport sector. At the same time, air pollution, mainly caused by pollutants such as fine particles, nitrogen dioxide and ozone, is responsible for almost 9 million premature deaths worldwide every year.

Rail transport offers undeniable advantages. According to a recent report by the European Rail Freight Association (ERFA,), transporting goods by rail consumes 6 times less energy, emits 9 times less greenhouse gases, creates 8 times less air pollution, and costs society 12 times less than road transport.

While rail remains one of the most eco-friendly transport modes, it still emits nearly 100 million tonnes of CO₂, globally each year, largely due to diesel, which accounts for 54 % of energy consumed (and even as much as 75% for freight).

How can we further decarbonise this mode of transport? Which technologies should replace diesel? And above all, how much will it cost?

The cost of decarbonising the railways

In our research, we propose a comprehensive methodology for assessing total social cost (TSC), including total cost of ownership (TCO) and total external cost (TEC). It's not just a question of looking at the direct costs of acquiring and operating rolling stock, but also integrating the externalities -indirect costs, which often remain hidden from public view.

The total cost of ownership includes two main components: investment costs, which cover acquisition, infrastructure, potential subsidies, and the resale value of rolling stock; and operational costs, such as energy, labor, network usage, insurance, and taxes. . These costs are ongoing and can vary depending on the intensity of use of the network.

Total external cost includes costs associated with climate change, air pollution and its impact on health, noise pollution (a major source of disturbance for local residents), accidents (causing material and human damage) and emissions generated upstream during energy production and manufacturing rolling stock.

Lastly, we include abatement cost in our analysis, a key indicator of the economic efficiency of various low-carbon alternatives. This cost represents the amount needed to reduce emissions by 1 tonne of CO₂. The lower the abatement cost, the more economically advantageous the alternative.

Several alternatives

We carried out a comparative study of four alternatives for a regional line in the Nouvelle-Aquitaine region, where trains currently run on diesel as the network has only been partially electrified.

The first option is to fully electrify the line, so that 100% electric trains can be used. Although this solution offers high performance and a significant reduction in emissions, it nevertheless requires a large initial investment. The second option involves using dual-mode electric-hydrogen trains. These trains can run in electric mode under catenary lines and switch to hydrogen mode on non-electric sections of the network.

The third option, a variation on the second, is to use dual-mode electric-battery trains. Finally, the last option we studied is trains that run exclusively on biofuel. This solution has the advantage of reducing the carbon footprint without requiring major modifications to the existing infrastructure.

Are dual-mode trains the right compromise?

Depending on the type of cost considered, the various solutions do not all offer the same advantages.

In terms of total cost of ownership, i.e. the cost borne by the rail operator, the dual-mode electric-battery alternative is the most advantageous, closely followed by the biofuel option. On the other hand, the fully electric alternative is relatively expensive, mainly because of the high charges imposed by the infrastructure manager for the use of the train path. Unlike the other alternatives, this option includes a special fee that allows operators to be billed for electrifying the line in proportion to their rate of use. The rail path also includes the cost of electricity, which, unlike diesel, hydrogen or biofuel, is not borne directly by the carrier. Lastly, the dual-mode electric-hydrogen alternative has a particularly high total cost of ownership, mainly because of the substantial investment required to acquire rolling stock.

Looking at total external cost, i.e. the cost of externalities borne by society, electric and dual-mode alternatives have a clear advantage as almost no atmospheric pollutants and greenhouse gases are emitted when operating these trains. Although biofuels are an improvement on diesel, they reduce these external costs to a lesser extent.

The results of this case study reveal a negative socioeconomic abatement cost for all decarbonised alternatives, which means they are beneficial for society. The dual-mode electric-battery alternative has the lowest abatement cost, despite requiring significant initial investment, indicating a balance between economic cost and socio-environmental benefits. The electric-hydrogen option has a higher societal abatement cost due to higher start-up and operating costs. However, it offers interesting long-term prospects, particularly in terms of reducing greenhouse gas emissions.

Electric trains also have a negative abatement cost, highlighting the economic suitability of this alternative as well. However, the cost is higher than the first two alternatives. As for biofuel, the results are mixed. Though the abatement cost is relatively low, the environmental impact remains significant in terms of air pollution and greenhouse gas emissions. Furthermore, ownership cost is comparable to that of diesel, which limits its economic attractiveness.

However, it is important to specify that the conclusions presented above only apply for the use case in question, which favours dual-mode solutions insofar as a large part of the line is already electrified. Since the network architecture and market structure of the geographical area studied have a significant influence on the choice of low-carbon alternative, it is important to analyse each case individually.