I bought train tickets in the UK like this one:
I always swipe them on the barriers1 before boarding the train, but I never see anyone else doing it. I wonder if it is necessary, but people simply don't care (or they have other kinds of passes), or if simply having the ticket is enough.
1 Barriers like those depicted below are already open when I board the train, which reinforces the idea that there could be another means to check the validity of tickets:
Context: I got used to validating public transport tickets in France, where simply having a ticket on board is not sufficient, e.g. some tickets can be used for different trains, so you need to validate ("composter") them before boarding. Since my UK ticket can also be used at different times, I wonder if they follow the same principle.