Diesel & alternative engines

Diesel traction engines power non-electric railway vehicles through mechanical, hydraulic, or electric transmission systems, providing traction on the approximately 44% of European railway lines that are not electrified.

Around 56% of European railway lines are electrified, and 81–82% of traffic runs on electric traction. Diesel traction covers most of the remainder: regional passenger services on non-electrified secondary lines, shunting operations at yards and terminals, engineering and maintenance trains, and freight on lines without overhead infrastructure.

In countries including Romania, Lithuania, Latvia, Estonia, Ireland, and parts of Poland and the Balkans, diesel remains the primary form of rail traction.

Diesel-mechanical transmission uses a gearbox between the engine and the axle. Diesel-hydraulic transmission, common in German DB Class 218 and similar locomotives, uses a torque converter.

Diesel-electric transmission — standard in most modern diesel multiple units (DMUs) and high-power locomotives — uses the diesel engine to drive a generator, which supplies electric traction motors; this arrangement allows regenerative braking into on-board resistor banks but not back to the grid.

Emission regulation

European diesel rail engines are regulated under Regulation (EU) 2016/1628, which established Stage V emission limits for non-road mobile machinery including rail traction. Stage V covers locomotive engines (category RLL) and railcar engines (category RLR) at all power ratings.

For locomotive engines, Stage V retained the existing Stage IIIB NOx and particulate matter limits unchanged, to avoid major redesign of locomotive systems. Railcar engines, by contrast, gained a new particle number limit alongside a reduced particulate matter limit.

Stage V applied to new rail engines from 2019, with RLL and RLR engines required to be on the market by 31 December 2020. Existing in-service engines are not required to be retrofitted.

Transition to alternative traction

Diesel traction is under pressure from the combination of emission regulation, operator decarbonisation commitments, and the availability of battery and hydrogen alternatives for non-electrified operation.

Battery electric multiple units are now the preferred replacement for diesel on routes up to approximately 100 km between charging points. Hydrogen is considered viable for longer non-electrified gaps where battery range is insufficient, though hydrogen supply infrastructure is not yet widely available in Europe.

Bi-mode vehicles — operating under the catenary on electric traction and on diesel or battery power off the wire — represent a compromise that avoids the infrastructure investment of full electrification while reducing diesel operation to the minimum necessary.

The European Alternative Fuels Observatory notes that 15–20% of European regional rail could run on hydrogen by 2035, though the pace depends on hydrogen production and distribution infrastructure that is not yet in place.

There is no EU-mandated phase-out date for diesel rail traction, in contrast to the 2035 ban on new internal combustion engine passenger cars.