How do you hop trains safely?

For passenger trains, stand behind the yellow line and wait for the train patiently.

If you mean freight trains, there is NO safe way to hop trains on a freight train.

Severly varied temperatures in different cars that are easily accessible to you will hurt you.

There's always a chance of a derailment, or a car that's defective, being set out in the middle of nowhere for weeks until a maintence train comes to repair it.

Freight cars are obviously NOT meant to carry people, and there's no reason to think differently, because one false move, and you're dead or seriously injured. Too many people have died trying to ride freight trains, being employees switching cars out from a yard to an industry or so-called hoboes, which are unjustly glorified tresspassers.

A lot of mentally unstable homeless people ride the rails, some have been known to be psychotic, and homicidal.

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There is no safe way full-stop. In the UK and much of Europe most rail freight is either carried in shipping-containers, bulk materials carried in special wagons. In the UK at least, modern train doors have locks so they can't be opened, from inside or out, until the train has stopped completely.

Oh, and BTW if you try it on as a means to evade the fare, the penalty is a heavy fine!