Diesel and alternative engines
Diesel traction engines power non-electric railway vehicles through mechanical, hydraulic or electric transmission, providing traction on the roughly 44% of European railway lines that are not electrified.
Diesel is the fallback of European traction and propulsion: under regulatory and commercial pressure, yet still the primary power source on lines from Ireland to the Baltics.
Unlike road transport, rail faces no EU-mandated end date for it.
Role in the European network
Around 56% of European railway lines are electrified, and 81–82% of traffic runs on electric traction. Diesel covers most of the remainder: regional passenger services on non-electrified secondary lines, shunting 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 engine and 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 — has the engine drive a generator supplying electric traction motors, an arrangement that allows regenerative braking into onboard 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 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. The European Alternative Fuels Observatory notes that 15–20% of European regional rail could run on hydrogen by 2035, though the pace depends on production and distribution infrastructure that is not yet in place.
Bi-mode vehicles — running under the catenary on electric traction and on diesel or battery power off the wire — avoid the infrastructure investment of full electrification while cutting diesel operation to the minimum necessary. 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.