This has come up before, and IIRC, the primary reason you can keep a train from starting with a penny on the tracks is railroad policy - if the engineer can’t get rolling in second notch on the throttle, they’re to stop and investigate the problem.
Sounds like a good problem to do over Saturday morning coffee… I’m sure I can find all this by googling.
Brainiac is pretty bad about this. Probably half the show is reused filler around segments. It’s really hard to watch multiple episodes in a row.
Well, nowadays they won’t be pure copper, but back at the time my uncle would have acquired this information, they were. If the choo-choo would be able to squash a copper coin (which I presume is easier than zinc), I’d be willing to call this a busted myth.
Addendum: I just called my uncle, to confirm the story. He said that, first of all, he only ever heard it about steam locomotives, and he doesn’t know whether it would work on a modern diesel. Second, he never actually personally saw it done; he just heard from others who claimed to have done it. So we’re well into FOAF territory here.