You would probably need to hot box the train pretty bad to get other people onboard high, so I'd hope the onboard ventilation isn't that bad, but it wouldn't surprise me!
Ultimately the driver is going to be the one who suffers the worst consequences if they make any mistake, so totally their call on wether they feel fit to drive or not.
I don't know why the hell anyone thinks smoking weed on a train is a good idea, especially with the whole no smoking/vaping thing. Vapourisers are becoming increasingly popular, so maybe it was one of those? Edibles would have probably been a little smelly, but fine. The thing is that skunk is very pungent, but the smell only gets stuck when you actually smoke/vapourise it, in that case the whole carriage will stink of weed for a long time!