At one time (before British Railways and Dr Beeching) rail companies used ticket punches with different symbols on different routes, so that a ticket inspector could see where a ticket had been used (in order to check that only authorised travel had been undertaken by the passenger).
The same ticket punches remained in use later on but, as they gradually wore out or became lost, rail operators replaced them with standard hole punches (of the type sold by office stationers), punches that stamped a date on tickets or some other types of device. Many though came to realise that any way of showing that a ticket had been seen would serve its purpose, so some conductors now simply scribble on tickets (or sign their initials). Occasionally a conductor might just look at a ticket (rather than punching it or writing on it), particularly when it's obvious that it couldn't be used again anyway. (I buy a day return ticket to the pub each Sunday evening, returning on the last train. It's obvious to a conductor that I couldn't use my ticket twice because there aren't any more trains on which it would be valid, so many conductors neither clip my ticket nor write on it).
These days though the majority of tickets used on many train services aren't paper ones anyway. Customers simply show the QR code on their phone app to the conductor, who scans it with his own phone app. The data is then fed to a remote computer, which notes that the ticket has been used for the relevant section of the journey, so that it can't then be re-used later on.