You may have wondered why some trains have multiple engines… and especially why some trains have an engine in the middle! I will do my best to answer these questions below.
Please note that I use “engine” as a synonym for “locomotive”, which is the technical term.
The Short Answer
Trains have multiple engines to provide more power to pull the train. Each locomotive has a certain amount of pulling power (called “tractive effort”), which is related to how many horsepower the diesel engine in the locomotive has. There’s a limit to how much tractive effort one locomotive can deliver, so railways connect multiple locomotives together to be able to pull more cars.
Multiple Steam Engines
In the days of steam engines, every locomotive had its own crew – an engineer to drive it, and a fireman to shovel the coal into the firebox to power the locomotive. (There was also a conductor and head end brakeman but they weren’t there to operate the locomotive itself, just the train)
As trains grew in length, railways built larger and larger locomotives. The famous Union Pacific “Big Boy” locomotives were one of the largest types, able to haul 3,600 tons of train up some serious hills. However, if you wanted to put more cars in a train than a Big Boy could haul, the only solution was to add more locomotives.
In steam days, this was called “double heading” (for 2) or “triple heading” (for 3).
Double heading was done, but it was awkward and expensive. It required a separate crew for each locomotive, costing the railways a lot of money to pay them. Also, this was before radios were used for train control, so crews had to communicate using the steam whistle to tell each other to speed up, slow down, or apply brakes.
Multiple Diesel Locomotives
When diesel locomotives started to be used, it wasn’t long before the railways wanted to connect them together to operate several locomotives as if it was one. This is called “MU” for “Multiple Unit”, and over time a standard evolved so most locomotive types can be mixed and matched together today. They don’t even have to have the same horsepower rating.
The engineer and conductor sit in the lead locomotive and control all locomotives from there. All locomotives in the train get the same throttle setting or brake applications.
This made it simple for railways. They could look at how many tons needed to be hauled, how steep the hills were on the route to be taken, then figure out how many locomotives were required to pull the train over the route.
There is a limit to how many locomotives can be added to the front of a train, but in general there can be up to six or seven without a problem. Most trains in North America will have at least two locomotives.
Some of the limits have to do with train handling, which leads to…
Why Do Some Trains Have An Engine In The Middle?
Some trains will have locomotives in the middle of a train, or even on the tail end. This is called “distributed power” and the locomotives are usually referred to as “DPU” (Distributed Power Units). The DPU locomotives have radio equipment in them so they can be remotely controlled, but otherwise they are usually normal locomotives that could have crew in them and could lead trains. There are some locomotives that have no crew cabs but they are rare.
Control equipment in the locomotive leading the train send commands to the DPUs via radio to increase or decrease the throttle, or to apply or release the air brakes. Most software used to control these remote locomotives can handle several DPUs at once at different parts of the train.
Why Use Remote Locomotives?
There are a couple of reasons to use remote locomotives instead of having all of them at the front of the train.
Trains often go around long curves, often at higher speeds like 40, 50 or 60 miles per hour. With all of the locomotives pulling on the front of the train, this leads to a tendency to pull the train straight through the curve (called “string lining“).
You can see this if you take a string, lay it on the floor in a curve, then pull on one end. The string tends to straighten out, and that’s what the train “wants” to do when all of the pulling is done from the front.
This can happen especially if there are a lot of empty, light cars with heavy, loaded cars behind them. Railways try to arrange cars in the train to avoid this.
One way to reduce the risk of string lining is to put some of the locomotives in the middle or end of the train, so that not all pulling is done from the front.
Trains use air brakes to slow or stop the train. Each car has air hoses on each end that connect together to make a continuous air line from the front of the train to the end. Air compressors in each locomotive add air to this air line. During cold weather, the connections between cars can leak and it’s hard to keep air pressure up on a long train. Having locomotives distributed through the train can help add air throughout the train to keep air pressure up.
Train brakes are applied by reducing the air pressure in the brake line. Each car’s brake equipment “notices” this reduction in pressure and applies the brakes in the car.
It takes time for the reduction in air pressure to travel through the brake line, especially for a train that could be two miles long. Brakes in the rear of the train could still be coming on while the locomotive has released the brakes and is accelerating.
Having radio-controlled locomotives throughout the train means that the reduction in air pressure can be done at different points in the train at the same time, so more brakes come on quicker.
One more question…
Why Do Trains Have Backward Engines?
This one is simple – locomotives can run forward or backward at equal speeds. The direction doesn’t matter, especially for diesel-electric locomotives.
See also Can trains run backwards?
It can be uncomfortable for the crew if the lead locomotive runs backwards, because the controls are set up in the cab to operate “forward”. The engineer basically has to look backward all the time while running the train. Imagine driving your car in reverse for several hours!
This is why you often see two locomotives “back to back”, so that if the engines have to be disconnected from the train, the crew can swap locomotive cabs and run the engines “forward” no matter which direction the engines are going.
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