Sorry to revive this thread, but hopefully this is the right thread for this - today I had the "privilege" of seeing 3 naughty kids cycling down almost the full length of a 700 unit I was on (between Dartford and Gravesend they did 3 "trips"), which is of course possible on long corridor trains. There were no staff on board (until 2 SE staff boarded at Gravesend, plus the driver of course). I sent a 61016 text about it around Swanscombe, and told the 2 SE staff who got on at Gravesend about it, who said they couldn't do anything (chances are the kids were off the train by this point anyway), but they said texting was the right action. I gave the number I communicated with details of the train on their request, so if it's enough of an offence they could find CCTV footage to identify them.
I say it as a disadvantage to long corridor trains, but of course that might be combatable with more staff management on board and catching people via the CCTV use, which I accept isn't always possible