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A locomotive provides the pulling or pushing power for a train. The three major propulsion families are steam, diesel-electric, and electric. Modern North American freight railroads are dominated by diesel-electric locomotives.
Diesel-electric locomotives
A diesel locomotive used on most large North American railroads is really a mobile electric power plant. Its diesel engine turns an alternator or generator. Electricity from that machine powers traction motors connected to the axles.
1. Diesel engine
Burns diesel fuel and supplies mechanical power. Railroaders often call it the prime mover.
2. Alternator
Converts the prime mover's mechanical power into electrical power.
3. Traction motors
Use that electricity to turn the wheels and create tractive effort at the rail.
Why not drive the wheels mechanically? Electric transmission gives a locomotive tremendous controllable pulling force at low speed without needing a huge mechanical transmission like a road vehicle.
Steam locomotives
A steam locomotive burns fuel to heat water in a boiler. The resulting high-pressure steam is admitted to cylinders, where it pushes pistons. Connecting rods transfer that motion to the driving wheels.
Steam locomotives are commonly described by their wheel arrangement using Whyte notation. A 4-8-4, for example, has four leading wheels, eight driving wheels, and four trailing wheels. Famous arrangements include 4-6-2 Pacifics, 2-8-2 Mikados, 4-8-4 Northerns, and articulated 4-8-8-4 locomotives such as Union Pacific's Big Boy.
Electric locomotives
Electric locomotives receive power from outside the locomotive, usually through overhead catenary or a third rail. Because they do not have to carry a large diesel prime mover, they can deliver high power with strong acceleration. Their biggest limitation is infrastructure: the route must be electrified.
Horsepower isn't the whole story
Horsepower describes the rate at which a locomotive can do work. Tractive effort describes pulling force. A locomotive needs both, but they matter differently. At low speed, adhesion and tractive effort are especially important; at higher speeds, available horsepower becomes increasingly important.
Railroad Central tip: Two locomotives with similar horsepower can perform differently because of gearing, traction-control systems, axle count, weight, adhesion and operating conditions.
Why do trains use multiple locomotives?
Long or heavy trains may require more power and tractive effort than one locomotive can provide. Multiple locomotives also help on grades and can improve train handling. Some are placed in the middle or rear of a train as distributed power units (DPUs), controlled from the lead locomotive. DPUs can reduce forces within a long train and improve braking and handling.
How to identify a diesel locomotive
Start with the builder, then look at the body shape, cab, radiator section, trucks, axle count and model-specific details. Railroad paint schemes can help, but locomotives are often sold or rebuilt, so paint alone does not prove the model.
| What to inspect | What it can tell you |
|---|---|
| Number of axles | Four-axle vs. six-axle road-switcher families |
| Cab and nose | Generation, safety-cab design and builder clues |
| Radiator / rear hood | One of the most useful model-family spotting areas |
| Truck design | Builder, era and locomotive application |
| Builder plate / data markings | Best evidence when visible |
Major North American locomotive builders
EMD
Electro-Motive Division became one of the defining builders of North American diesel locomotives. Well-known families include GP, SD and F units.
GE / Wabtec
General Electric became a dominant freight-locomotive builder. Wabtec acquired GE Transportation in 2019 and continues locomotive production and modernization.
ALCO
The American Locomotive Company was a major steam builder and later produced distinctive diesels before ending U.S. locomotive production in 1969.
Baldwin
Baldwin Locomotive Works was one of the great steam-era builders and also entered the diesel market.
Lima
Lima Locomotive Works is remembered for powerful steam locomotives and the Shay geared locomotive lineage.
Modern rebuilders
Today's locomotive industry also includes extensive rebuilding, remanufacturing and emissions-upgrade programs that can make identification more challenging.
Locomotive terms worth knowing
Prime mover: The main diesel engine in a diesel-electric locomotive.
Traction motor: Electric motor that provides torque to the locomotive's axles.
Tractive effort: Pulling force a locomotive can exert.
Adhesion: Grip between wheel and rail.
Dynamic braking: Uses traction motors as generators to help slow the train; the generated energy is typically dissipated as heat on conventional diesel-electric locomotives.
Road switcher: A versatile locomotive designed for road service while retaining the visibility and flexibility useful for switching.
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Ask the ConductorFeatured Locomotive Profiles
Explore detailed guides with real railroad photography, specifications, history and identification clues.
EMD SD40-2
3,000 hp • Six axles • Dash 2 legend
Explore EMD SD40-2 →
EMD GP38-2
2,000 hp • Four axles • Long-lived road-switcher
Explore EMD GP38-2 →
EMD F7
1,500 hp • Streamlined classic • A and B units
Explore EMD F7 →
Union Pacific Big Boy
4-8-8-4 • Giant articulated steam power
Explore Union Pacific Big Boy →
GE ES44AC
4,400 hp • AC traction • Evolution Series
Explore GE ES44AC →
EMD SD70ACe
4,300 hp • AC traction • Modern EMD
Explore EMD SD70ACe →Photography credits and Creative Commons license links appear on each locomotive profile.
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More Locomotive Profiles
GE AC4400CW
A 4,400-horsepower six-axle AC-traction locomotive that became a heavy-haul mainstay across North America.
Explore GE AC4400CW →EMD GP9
A 1,750-horsepower four-axle road-switcher that became one of the classic first-generation diesel locomotives.
Explore EMD GP9 →
ALCO PA
One of the most dramatic streamlined passenger diesels ever built, remembered for its long nose, distinctive profile and ALCO sound.
Explore ALCO PA →DEEPER REFERENCE
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