I agree with the "experience" part. I once had a class from Ysgol Dolwyddelan (a small village primary) board for a cricket tournament in Llanrwst. There wasn't really any "teaching", the ticketing discussion (TfW's free accompanied child concessions) took place just between me and the group leader), but the simple activity of travelling by train to get somewhere makes it a bit more familiar and will encourage use later in life.
Though I think this highlights another issue with adding it to trying to shoehorn it in. If it's just naturally part of an activity, the pupils get to actually do it. If it's to fit into an hour's PSHE lesson in the middle of the school day, is it really feasible to take pupils down to a bus stop, get them all to buy a ticket/scan their pass on the bus, travel a couple of stops to see how to know where to get off, wait for a return bus in the opposite direction, do the same again, etc? With catching a train, getting a class of 30 children to buy a ticket somehow and the scheduling of the train services would be even more challenging, except perhaps those whose school is outside an Underground station somewhere fairly central. Doing it in a classroom would be half-baking it.
The young people I volunteer with do travel by public transport at times and as a result they naturally learn how to do it, but I would shy away from specifically encouraging groups to go out of their way to teach it as it's not practical, or at least, there's a thousand other things they'd be better spending their time doing.
This thread was a spinoff from a discussion regarding railway ticket complexity and fare evasion though. Bringing it back to that, the fares system for GB rail is so complex that the forum occassionally runs fares workshops which take longer than a PSHE session would and cover reasonably advanced topics. That many of the advanced fare topics even exist is more likely to be a problem for people making their first rail journey than the fact that buying a ticket and boarding a train isn't taught in schools. And for those living in Wick, you'd have to explain why their national operator's mobile app requires you to search for WCK instead of Wick, because if you search for Wick, none of the results shown are indeed for Wick.