Assume you can hook every locomotive and car Union Pacific owns up into a single gigantic freight train. Safety and laws aside, would it work?
In: Physics
Assuming we can disregard things like safety procedures, conditions of the track and rolling stock, fuel for the locomotives, assembling this monstrosity, etc…
There are two primary things that limit the length of a train. Those are:
1. The amount of tractive effort the locomotives can supply
2. The strength of couplers between cars
Trains aren’t frictionless. They go through a lot of effort to reduce friction in every place they can, but you can never escape it. So the weight of the train does add up. At some point, the weight of the train becomes so great that the locomotives pulling it cannot put enough power down into the track that it can be pulled.
Do note that the amount of power an engine can make is not necessarily equal to the amount of tractive force it can supply. The fact that railroad designers have gone far out of their way to make friction as low as possible on rails does work against the engines themselves. The friction between the wheels of the locomotives and the rails is usually the limiting factor here; if a loco pulls harder than the friction allows, the wheels just slip and the whole train goes nowhere.
So to pull an arbitrary-length train, you need some proportion of locomotives pulling.
The other thing that matters is where the locomotives actually *are*. If you put them all in the front, and trail all your cars behind, you could snap the train apart. Think about it: if you had a near-infinite amount of train cars to pull, and a near-infinite amount of locomotives to pull them, all of that force needs to transfer through the first coupler between all the locos and all the cars. That would need to be a near-infinite strength coupler. That obviously doesn’t exist
To eliminate this, you need to pepper your locos through your train, interspersing them at regular intervals to ensure that no coupler gets overstressed. Real trains already do this, and the technique is often called “distributed power”. It’s why if you wait at a level crossing for a freight train to pass, you might find locomotives at the rear end or in the middle of the train. That’s what they’re doing.
With a high enough ratio of locos to cars, and with distributed power throughout the train, there’s basically no limit to how long a train can be. If you clipped the train apart at every locomotive, you’d have a bunch of short trains that can all run on their own, With this arrangement, you’ve essentially created a bunch of short trains that all happen to be linked together.
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