Having had a drink is one thing.
If you get wrecked and make a nuisance of yourself, then you're liable to attract the wrong kind of attention and get removed, usually by the police rather than an RPI.
Anyone from a platform supervisor/dispatcher to guard to RPI etc can enforce byelaw 4 however.
I had some lad roll up on my platform on the bank holiday morning a few weeks back, and for no reason as he walked past, obviously a bit worse for wear, called me a w****r.
I then kicked him off the station and wouldn't let him back in until he'd sobered up/missed a good few trains to his destination and been delayed for a couple of hours, as his behaviour was unacceptable, so I decided at my discretion to enforce the byelaw. I'm not remotely interested in helping drunks out at 8am.
Conversely, I had a bloke puking in the bogs, but generally being apologetic and embarrassed and apart from that being in a reasonable sort of state. On that occasion, as it was Saturday night, myself and a passing BTP officer got him a coffee, sat him down for half an hour, and made sure he got on his train safely and on his way once he started sobering up a bit.
Good natured, happy people just having a night out and not causing anyone any harm will received nothing but good humour from me in return.