How Do Locomotives Work?

You may have seen a train locomotive roll by, pulling a long train, and wondered how the locomotive actually works. This article will explain the different types of locomotives and how they work.

Types of Locomotives

There are essentially two types of locomotives: steam or electric. Many people know what a steam locomotive, or steam engine, is: a hulking, noisy engine that burns something to make steam which somehow moves the wheels using a set of rods. Plumes of smoke and steam come out the top and sides.

Electric locomotives are very different. Large motors turn each wheel to make the locomotive move. These are a lot less visible than the steam coming from a steam locomotive, but they do the same thing.

Electric locomotives can be broken down further based on where the electricity comes from:

  1. “Pure” Electric (power comes from the rails or from overhead wires)
  2. Diesel-electric
  3. Gas-electric
  4. Gas turbine-electric
  5. Battery powered

Most of this article will talk about the various types of electric locomotives. But first… steam engines.

How Does a Steam Locomotive Work?

At its most basic, a steam locomotive has two parts: a steam generator, and cylinders and wheels to make it move.

Steam locomotives have a boiler, with a firebox that heats the water in the boiler until it turns into steam. The steam is then piped to cylinders on each side of the locomotive, which drive the wheels by connecting rods.

Making Steam

Anything flammable can be used to make the fire for the boiler. In the beginning, steam locomotives burned wood, which was often nearby and easy to obtain. Eventually wood became harder to obtain, and locomotives became larger and it was impractical to keep shoving logs into the firebox. Most locomotives burned coal, which was stored in a tender (auxiliary car) attached to the locomotive (or in the locomotive itself if it didn’t have a tender). Eventually railways converted many of their steam locomotives to burn oil, as it was easier to handle.

Fire is one part of making steam. The other is obviously water! Steam locomotives obtain their water from nearby bodies of water, wells, fire hydrants… wherever it is available. As locomotives became more advanced, the quality of the water became very important. Steam locomotives now require treated water to reduce corrosion and other issues. Since the steam is used once and then exhausted into the atmosphere, steam locomotives need a huge amount of water and need to get more water frequently.

Using Steam to Move

A series of valves allow the steam from the boiler to enter cylinders on the side of the locomotive. The steam pushes the cylinder, and the valves move to let the condensed steam escape the cylinders and up the smokestack.

The cylinders are connected to large driving wheels on the locomotive via rods. Most steam locomotives had two cylinders, but some locomotives had three or four.

Steam locomotives often had smaller wheels at the front and/or back of the locomotive to help guide the locomotive and support its weight.

“Pure” Electric Locomotives

electric locomotive in Pisa, Italy
Electric locomotive leading a train in Pisa, Italy, 2022.

Electric locomotives use power from outside the train. This power may come from overhead lines (like a trolley or the locomotive shown above) or from the rails (like a subway train), or a combination of the two.

For example, the photo above shows an electric locomotive with a “pantograph” raised to touch the wire above the train. The wire provides the positive charge and the rails are the ground (“earth”) side of the circuit. In some cases there may be a third rail between the normal two rails that provides the positive charge, instead of an overhead wire.

Electric locomotives have motors that drive each wheel on the locomotive. These are called traction motors. Not every wheel has to be powered; some axles / wheels may not have a motor and instead just roll along like a freight car axle. These unpowered axles are often provided to help spread the weight of the locomotive across more wheels to help prevent damage to the track from a heavy locomotive.

Diesel-Electric Locomotives

Diesel-electric locomotive on a GO Train in Toronto, Ontario.
Diesel-electric locomotive on a GO Train in Toronto, Ontario.

We often talk about diesel engines or diesel locomotives powering a train. In the vast majority of cases, these are actually diesel-electric locomotives. There is a very large diesel engine “under the hood” which drives a generator or an alternator to make electricity. This electricity is then sent to the traction motors to make the locomotive move.

The diesel engine is usually an engine specialized for trains. In some cases the same type of engine has been used to power boats. These diesel engines are much larger than those in trucks.

Diesel engine inside an F40PH-2 locomotive
Diesel engine inside an F40PH-2 locomotive

Example: VIA Rail’s F40PH-2 locomotives power many of VIA’s long distance trains. These locomotives have a 16-cylinder EMD 645E3 engine which can produce 3,000 horsepower (HP). At one end, a generator is attached to the crankshaft and turns when the diesel engine is running. The spinning generator produces direct current (DC), which is used to drive the wheels and also to power the lights, heat, and other electrical needs on board the train.

AC versus DC

DC generator inside an SD40 locomotive
DC generator inside an SD40 locomotive

The diesel engines in these locomotives have either an alternator or a generator. An alternator produces AC (alternating current) and a generator produces DC (direct current).

Most older diesel-electric locomotives had generators and traction motors that used DC. This was simpler and cheaper but had some issues. Locomotives with generators usually have 8 throttle levels with set power output from each level. You may hear someone talk about “run 8” which is the eighth (or maximum) power setting… equivalent to “flooring it” in a car.

The majority of new diesel-electric locomotives today use AC power from alternators, and variable frequency drivers (VFDs) in the traction motors. These are more expensive but are simpler to maintain and allow the locomotive to pull more freight, especially at lower speeds, because there is less “wheel slip”. This article talks more about AC versus DC traction in locomotives.

Gas Electric Locomotives

Self-propelled gas-electric passenger rail vehicle
CP 9008 with passenger trailer

Gas electric locomotives operate similarly to diesel-electric locomotives, except that they have gasoline engines instead of diesel engines. Today gas-electric locomotives are very rare. In the early part of the 20th century, gas-electrics were more common, especially in self-propelled passenger vehicles.

The development of the diesel engine in the 1920s and 1930s made gas-electric locomotives obsolete, as a diesel engine produces more torque than a gasoline engine, which allows locomotives to pull more freight.

This article from the October 1925 CN “monthly bulletin” for employees discusses the “Oil Electric” passenger vehicles. These burned oil in an engine to drive a generator, which produced DC power to drive the traction motors attached to the wheels. These cars were capable of towing a “trailer”, or unpowered passenger car, to carry additional passengers. Cars like these were often known as “doodlebugs”.

Gas Turbine-Electric Locomotives

There have been several attempts at using gas turbines on board locomotives to provide power to the traction motors. The most famous gas turbine locomotives in North America were the 55 operated by Union Pacific between 1952 and 1970. These were called GTELs, for Gas Turbine Electric Locomotive.

These very large locomotives had a gas turbine, similar to an aircraft engine, which drove a DC generator. The last generation of these locomotives were monstrous, weighing in at 424 tons (384 tonnes) and producing 8,500 horsepower – which still stands as a record for a single locomotive “prime mover”. The turbines burned Bunker C oil, which was preheated on board with a steam generator so it would flow better.

They were a successful locomotive but proved inflexible compared to using multiple diesel-electric locomotives, and the cost advantage of using Bunker C waned over time.

There were experiments using coal to power gas turbines but they were unsuccessful.

Battery Powered Locomotives

Battery powered locomotives have a series of batteries on board that store energy. When needed, some of this energy is sent to the traction motors. They operate the same as a diesel-electric locomotive except that there is no diesel engine on board; the battery must be charged from another source before the locomotive starts moving.

Battery powered locomotives were developed long ago. The first known battery powered locomotive was built in 1837, according to Wikipedia. This type of locomotive was preferred for certain industrial applications where an ignition source like a gas or diesel engine was dangerous, such as mining.

The main issue with battery powered locomotives is limited range. Improvements in battery technology over the decades have allowed greater range, but even the new GE battery-powered locomotive can only operate for 30 minutes at full power.

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