Rail traction: Engineering methods

Wire, diesel or battery: three ways to pull a train

What this tab covers The engineering methods that can do this conversion, compared on efficiency, scale, economics, limitations and reliability.

Two main ways to drive a train today: take electricity from wires, or carry a diesel engine that makes it on board. Battery and hybrid locomotives are a growing bridge between them.

Comparison

Method	Efficiency	Scale	Economics	Limitations	Reliability
Electric train and metro (overhead line or third rail)¹³⁴¹⁴	about 80% wire to wheel for traction (calculated); 43.4% average in the Turkish study, whose wider boundary is not comparable	Transformers 1.5-5.2 MW; traction chains from a few hundred kW to over 7 MW; 392,481 route-km electrified	GBP 0.75-2.5 million per single-track km to electrify (UK); over 35% cheaper to run than diesel	High fixed cost; only pays on busy lines; needs grid connection	No availability figures in sources used
Diesel-electric locomotive³¹⁰	about 29% fuel to work (Turkish average)	4,400 hp class freight locomotives; over 10,000 on Wabtec's EVO platform; nearly 1,200 EVO Tier 4 units	No purchase price in sources used; engine upgrades save up to 7% fuel	Exhaust emissions (US Tier 4 requires 76% less NOx and 70% less particulates); fuel cost	EVO platform in service since 2005
Battery and hybrid locomotive¹¹¹²	about 11% fuel saving in a mixed consist; up to 30% expected for larger batteries	2.4 MWh pilot (2021); 7 MWh units commissioning at BHP (2026)	No price in sources used	Battery mass and cost; range limited without frequent charging	Early stage

Electric traction wins on efficiency (about 80% wire to wheel against about 30% fuel to work for diesel), on local emissions and on power, since the train does not carry its engine. Diesel wins where traffic is too light to pay for wires, which is why the Americas, with long, lightly used freight lines, run almost entirely on diesel. The Turkish study's 43.4% for electric against 29.1% for diesel uses fleet-level national data and a boundary the abstract does not define, so it sits well below ABB's on-train figure. Battery locomotives aim to capture braking energy that diesel-electrics now waste in resistors.

How each method works

Electric train and metro (overhead line or third rail)

Current from the line passes through a transformer (on AC), converter and traction motors. Distributed motors under many cars (multiple units) or a single locomotive pull the train; braking returns power to the line.

Diesel-electric locomotive

A diesel engine of several megawatts drives an alternator; the output is rectified and inverted to power the axle motors. Dynamic braking dissipates energy in resistors.

Battery and hybrid locomotive

A large battery replaces or supplements the engine, charged at the yard and by regenerative braking; works in consists with diesel units or alone on suitable routes.

Sources

Traction systems for high-power applications (brochure, Rev B) (company data) ABB, 2018
Energetic, exergetic, and environmental evaluation of railway transportation by diesel and electric locomotives (Environmental Progress and Sustainable Energy 41(3), doi 10.1002/ep.13804); Turkish State Railways freight data 2005-2018 M. E. Akay and A. Ustaoglu (Wiley), 2022
Railway Statistics Synopsis, 2025 edition International Union of Railways (UIC), 2025
Advantage Wabtec: relentless innovation drives Evolution Series engines into the future Wabtec, 2025
Wabtec: battery-electric locomotive reduces fuel use by 11% FreightWaves, 2021
E-quipment highlight: 4,400 hp Wabtec FLXdrive electric locomotive Electrek, 2026
Electrification Cost Challenge Railway Industry Association (RIA), 2019