Infinity Train | Theory | Who Built the Train? | I Called It! (Maybe) 

Okay! After drawing from my limited knowledge and doing a little research, I've come up with a possibly plausible but mildly ignorant theory for the train's origins!
I'll add that for all we know, Tulip was directly / indirectly responsible for the train's creation. It would be a little strange for technology to advance that much in her lifetime, unless she's just that smart, or discovered some form of reality-warping science that made the train possible ... and caused the apocalypse.