I learned how to do it in the brakeman training that the PLA did for crew on the Niles Canyon Railway. A key rule we had was the train had to be moving at 6 mph or less. Above that and you would get yanked as you grabbed the grab iron. When you do it right it seems like a very smooth and natural motion. Ironically the weekend warriors were safer at doing it than the professionals. I would guess it was because we didn't do it everyday so we did it by the book. On the attached video you see me get aboard the caboose at the 6:10 mark.
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