I think you may have misread my post by mistake.
I'm not referring to travelling on the train; I'm referring to exiting the station.
Let's go back to my example, which I'll try and put into better words.
My parents and I are making a journey from say, Newport to Radyr. I have a 16-25 railcard. On the train going to Cardiff, the conductor comes round and checks all of our tickets, including me showing my railcard. The conductor then marks the tickets, the job is done and that is that.
Then say I went to leave at Cardiff instead of continuing to Radyr, i.e. stopping short. I don't want to take lots of bags with me, so I just take my phone, maybe some cash and my marked ticket to exit the station, to the point where I will no longer be using the railway from this point onwards.
I would obviously have a shock if I come across a Revenue Protection Inspector asking to see my railcard when it has already been checked ON BOARD THE train, as I thought that was it.
If I ever was in such a situation in real life I obviously would keep my railcard on me until I left the station premises now as Revenue Protection are present at Cardiff.
I'm well aware of crew changes. I've caught a train from Newcastle to Bristol with THREE crew changes, with ALL THREE members of staff checking tickets.
I'm quite sure I read your post correctly. I thought you were making the point that once you'd had your ticket checked once it didn't need checking again. I don't see why it's different if it's checked twice whether that's on the train or not.
I realise that such gates can slow things down - I used to exit Kings Cross much more quickly before the current barriers were installed especially when I didn't have to fiddle with my tickets when I have a suitcase and backpack, but I realise they're quite entitled to put them in.
