Why do freight trains have multiple engines at the front and rear?

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I always wondered what the extra locomotives are for on modern freight trains, are they for extra power?

In: Engineering

I guess for the same reason as with other trains, for extra power and so you don’t have to turn the entire train around when you want to go backwards.

Each locomotive has a power rating which essentially means how much weight it can pull. The total size of the load will dictate how many locomotives are needed to pull the train. The terrain is also taken into account, since it takes more energy to haul the load uphill.

When multiple engines are connected, in physics it’s known as a connected body. In connected bodies, to calculate the force, you add the masses of all the objects then multiply by the acceleration. So the more mass that is accelerating directly effects the force that the train can pull

([Train1 mass] + [Train2 mass]) * acceleration

If you have 2 identical engines connected, it’s the equivalent to one at double the acceleration

You could have fewer more powerful engines, but the problem is applying the power to the track and not having the wheels slip and spin with more engines the power is distributed across more wheels and the traction is greater.

Already some good answers here but there’s one more to consider: train slack and load on the couplings. If we have a 250-car train needing 4 engines and put them all at the front, that’s a lot of load on the couplings toward the front and also a lot of slack which is hard on those couplings. If you instead do 2 in front pulling and 2 in back pushing you can control your slack a whole lot better. Even 2 in front and 2 in the middle is better than doing all 4 up front.

Also, if they just used a single really big engine that could handle the highest load ever needed, most of the time a lot of the capacity would be wasted. Breaking that into a few smaller engines allows you to match the pull to what’s currently needed, for cheaper average engine cost and overall fuel efficiency.

Wow, thank you all for the great and clear answers (even the physics one!) Considering I know nothing about physics!

They always try to give technical reasons like “power” and “load.” The real truth is that it is a result of the Engineers Employment Guarantee (EEG) of 1948 which requires multiple engines and therefore multiple engineers on every train that travels in excess of 50 miles. It was negotiated by the Train Workers Union (TWU). It’s twu, it’s twu!

While I’m not a train expert by any means, I can imagine redundancy would be a valid reason as well. If one of the locomotives breaks down, the train is at least still able to keep moving as opposed to getting stuck. After all, it’s not like a train can just go around another train that’s stuck on the rails.