Is there a physical limit for the length of a train?

Question

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

Answer 1

The length of the train introduces some complex dynamics. There’s a small amount of play between each car, and each car has momentum.

It’s basically a long, heavy chain. You can’t push a chain. It just bunches up. You can only pull.

When you have a long train, you can have one part of it going up a hill and another part going down a hill. The cars on the downhill side want to accelerate, the cars on the uphill side want to decelerate.

If the front is climbing the hill and the rear is coming down the hill, what happens to the middle? It gets pushed by the downhill cars into the uphill cars.

If you apply the breaks to the locomotives in front and the cars behind it, what happens to the cars behind them? They risk running right into the decelerating cars and hopping the tracks.

Now expand that to every single car in the US combined into a mega train.

There are 1.6 million rail cars and 28,000 locomotives in the US . Combined they form a single train over 18,000 miles long. For reference, a flight from LA to NYC is only 2,445 miles.

So what happens when we have a 18k mile long train climbing up and down the Rocky Mountains and the Appalachians and the Sierra Nevadas? Even if we somehow found the perfect rail path that allows for a continuous 18k mile long train without running into itself, it’s going to derail all over the place. The train will need to slow down in areas, and any mistake will cause a cascade of decelerations back to the point where the train cars just can’t stop in time, and they’re pushed off the tracks.