Yep, looks like he was using a train while paying for a proper ticket. Not a bug indeed. Also, certain boat routes also cost similar sums, per trip.
For the sake of completeness, though: there is a certain hacker in the game which, if you've done certain things right for him - will provide forged train tickets, which remove this particular expense altogether. And he'll regularly have more of 'em for a very token price, iirc. However, he's plenty hard to even ever meet, and hard to deal with, and his forged tickets come with a small chance to be detected as forgery (couple percent chance or so) every time one is used - and if it ever happens, then iirc one won't be able to use trains ever again. So his forged tickets are not really worth it, IMO. At least one other thing that guy does, though - is really dope, so you may end up using his tickets as well if you go work for him for other thing(s) he provides. Potential profit may end up being many hundreds coins outta using these forged tickets, if you're lucky enough. YMMV.