The Race to Replace Diesel: How Hydrogen and Battery Trains are Decarbonizing Rail

For decades, rail has been celebrated as the poster child for clean transportation. Yet, behind the sleek high-speed electric networks lies a dirty secret: a surprisingly large share of regional rail services still runs on fossil fuels. Across the European Union, only about 57 percent of the railway network is electrified, with countries like Italy falling below one-third. For many rural and regional routes, stringing continuous overhead power lines is simply too expensive and technically challenging, leaving diesel locomotives as the stubborn fallback.[6]

That paradigm is now changing rapidly. As climate mandates tighten and the cost of carbon rises, the rail industry is in the midst of a massive propulsion shift. The goal is to achieve zero-emission traction without the staggering infrastructure burden of full electrification. To bridge this gap, engineers and transport authorities are turning to alternative green technologies that carry their power with them.[1][8]

Two primary contenders have emerged to dethrone diesel on these non-electrified routes: hydrogen fuel cells (often referred to as "hydrail") and battery-electric multiple units (BEMUs). While they compete for the same contracts, both are fundamentally electric trains. They feature the same smooth acceleration, regenerative braking, and quiet electric traction motors. The difference lies entirely in how they store and generate their electricity onboard.[4][7]

The mechanics of a hydrogen train are a marvel of modern electrochemistry. Instead of burning fuel in a combustion engine, a hydrogen train carries roof-mounted tanks of pressurized hydrogen gas alongside a fuel cell stack. Inside the fuel cell, the stored hydrogen is combined with oxygen drawn from the ambient air. This electrochemical reaction generates a steady flow of electricity, with the only direct byproducts being heat and pure water vapor.[7]

Because fuel cells operate most efficiently at a steady, continuous output, they are almost always paired with a lithium-ion battery pack. This hybrid approach is crucial because rail traction is highly dynamic. The battery handles the sudden peaks in power demand—such as accelerating out of a station or climbing a steep gradient—while the fuel cell provides the cruising power and recharges the battery. The battery also captures and stores energy during regenerative braking, maximizing the train's overall efficiency.[4][7]

The pioneer of this technology was French manufacturer Alstom, whose Coradia iLint became the world's first commercial hydrogen-powered passenger train. It proved that hydrogen could match diesel's operational range, with one unit famously covering 1,175 kilometers on a single tank. By 2022, fleets of these trains were entering regular passenger service in the German regions of Lower Saxony and Hesse, logging hundreds of thousands of kilometers and generating immense optimism for a hydrail revolution.[1][5]

But scaling the technology from demonstration to daily regional service revealed significant growing pains. In 2024 and 2025, those early German fleets faced a cascade of operational hurdles. Operators reported fuel cell degradation that ran ahead of the designed service intervals, software faults affecting energy management, and severe reliability problems with contracted hydrogen suppliers. The logistical reality of keeping high-pressure gas flowing to regional depots proved far more complex than anticipated.[5]

These setbacks forced some German transport authorities to temporarily pull their hydrogen units and rely on leased diesel backups to maintain timetables. The experience highlighted a hard truth: hydrogen rail is not a plug-and-play replacement for a diesel locomotive. It is a complex, region-wide systems project. Success requires not just a working train, but a flawless ecosystem of green hydrogen production, specialized storage, rigorous safety protocols, and robust refueling infrastructure.[4][5]

While hydrogen stumbled over infrastructure hurdles, battery-electric technology advanced at a blistering pace. Benefiting from billions of dollars of research in the automotive sector, battery packs have become lighter, cheaper, and more energy-dense. BEMUs offer a compellingly simple value proposition: they have fewer moving parts than fuel cells, require no high-pressure gas logistics, and can plug directly into the existing electrical grid.[4]

Battery trains excel under a deployment model known as "partial electrification." Instead of wiring an entire 100-kilometer route, a transport authority can electrify just the busiest segments or install fast-charging overhead rails at terminal stations. The battery train charges while running under the wires or waiting at the platform, then operates seamlessly on battery power through the non-electrified gaps. For many regional networks, this hybrid infrastructure is the most cost-effective path to zero emissions.[1][4]

Consequently, the competitive boundary between the two technologies is shifting. As battery ranges improve, the number of corridors where hydrogen is clearly superior is shrinking. Several transport authorities that initially explored hydrogen have pivoted, announcing that future orders for short-to-medium regional routes will focus on battery-electric multiple units instead.[4][5]

However, hydrogen is far from obsolete; rather, it is being repositioned as a highly targeted tool. Hydrogen still wins decisively on long, remote, or heavy-duty routes where charging opportunities are non-existent, or where the sheer weight of the batteries required would make the train too heavy to operate efficiently. Where timetable intensity makes battery charging impractical, hydrogen's fast refueling time remains a critical advantage.[1][4]

Recognizing this, several European nations are doubling down on hydrail for specific use cases. Italy, for instance, has committed €300 million to replace diesel trains with hydrogen units across six regions by June 2026. Crucially, the vast majority of that funding—€276 million—is dedicated not to the trains themselves, but to building the localized production, storage, and fueling infrastructure required to make renewable hydrogen reliable.[3]

Similarly, Siemens Mobility is moving forward with the rollout of its Mireo Plus H trains in Bavaria, scheduled for 2026. These two-car trains, ordered by the State of Bavaria, feature roof-mounted fuel cells and 1.7 megawatts of drive power, enabling a top speed of 140 km/h. They will replace diesel units on a 32-kilometer non-electrified route, supported by a dedicated electrolysis plant built by Deutsche Bahn to ensure a steady supply of green hydrogen.[2]

Recent digital modeling by researchers at the Sapienza University of Rome confirms that there is no universal winner. By simulating hybrid train performance on real railway lines, they found that flat, short routes favor larger battery configurations to smooth out power fluctuations, while mountainous or long-distance routes require a heavier reliance on hydrogen fuel cells to ensure stable operation without excessive battery wear.[6]

Ultimately, the future of non-electrified rail will not be a winner-take-all battle between hydrogen and batteries. It will be a tailored, route-by-route ecosystem. Transport authorities will deploy battery trains for commuter hops and partially wired regional lines, while reserving hydrogen for the long, heavy hauls into the hinterlands. Working in tandem, these two technologies are poised to finally consign the diesel locomotive to the history books.[1][8]