Is it possible to walk on top of a moving train?

We've all seen it in countless movies, if there's a train scene then there's a 99% chance someone will walk on top of it. Now many people seem to struggle enough walking inside a train without grasping the seats as they pass, so how realistic is it to walk on top of one?
I reckon with windgust and vibration it's not as easy as Hollywood makes it out to be.
Is it possible to walk on top of a moving train?
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