Getting back to the Christmas carol - the lines about the train are not meant to indicate that there is no hope, but rather they are giving a warning, in the form of a flash forward: there is hope still, but you better get moving, because SOON that train is going to be gone. (These are NOT just a random bunch of words that defy meaning. However, you cannot expect to always find linear, narrative logic in African-American song).

See, btw, 'This train', 'People Get Ready' - there are probably others of this ilk, if you look for them.

I think it quite likely that this carol originally had non-Christmas lyrics, and someone decided to put Christmas lyrics to it, but retained the train refrain, because - why not?