In the real world, of course, what happens is that they land on the spot and their feet immediately get pulled from under them, and they get thrown forcefully onto the floor of the train, performing several involuntary somersaults until the rear wall of the compartment summarily halts their progress.
Winner!
Beat me to it! I’m going down to the quarry and throw some shit down there.
Disregarding the effect of wind (and such things as low bridges & wires) this is incorrect - a jump performed on top of the train would be no different from one performed inside the train, or one performed on a train that was not in motion.
How will you do that?
If such a jump were performed while in a movie I believe that a low bridge would somehow become involved.
And then, despite having jumped before reaching the bridge, you would smash into the bridge and fall back on the roof of the train much further back. Unless you were on the last carriage in which case you would fall onto the tracks, look up to see the train heading into the distance, and be struck by the following handcar.