Maybe, just maybe, it's a percussion effect and doesn't
mean anything. It's timed to underscore the word Villians, so I should think it is a purely musical decision.
That's not to say he didn't use music for literal effects sometimes, railroad hammer's, heartbeats etc. But not in this case IMO
It's all open to interpretation though, and if you hear it as a train, then to you it's a train.
