Complex sentence: The brake-man runs up to the engine to tell Casey about the oncoming double-headed slow freight train, which is coming, but fails to get the message, then jumps off the train after saying' So long.
Split into simpler sentences: The brake-man runs up to the engine to tell Casey about the oncoming double-headed slow freight train, which is coming, but Casey fails to get the message. Then the brake-man jumps off the train after saying' So long.