There are no accessible buttons or butterfly valves in the 313-the cab is locked separately and there is nothing in the vestibule that can be accessed without a carriage key.
I don't know if the 313s are different from 314s, but in the vestibule you still have access to local door switches and open/close buttons?
As for a 317 you are correct with the butterfly valve for the door-although this won't put the brakes on, only loose interlock. None of the controls bar the emergency brake will do anything without a master key on - same on any MU.
Which interlock, traction or door? I'd imagine it was 'door' in which case I'm pretty sure the brakes would come on (as indeed they should, thats a serious high risk incident if a door can open in motion without brake intervention). There have been cases where delays have been caused by a not-fully-closed crew valve giving unsolicited brake applications.
My point is still being missed though-there is damage you could do in there with isolation cocks and MCBs etc if you really wanted to but to be honest if you have the keys to access them in a cab then you can also access them within the coaches-isol cocks under seats, MCBs in body end cupboards etc.
True, but they're not laid bare for anyone to see in the saloon. Not many people would know that ASIC or ASM stands for, nor think that there was an iso cock inside it.
I am not saying its fine to travel in the cab but saying that we are talking here about commuters doing it who are just trying to get to work and arnt there to cause problems-we are not talking about criminals intent on damage who will find a way to get into those areas if they are determined enough no matter what the deterrent.
And yes, I am actually a driver and I don't see why people are so shocked that I would be happy just kicking people out of the rear cab and wouldn't demand the train is held up whilst I get the police to attend and try to make the passenger wait with me for them when they will just get a slapped wrist and I will most probably be asked to justify the delays etc.
Personally I'd rather hold a train up for a few minutes than be asked the awkward questions after a cab is trashed or a senior TOC manager spots it happening: "how long have you allowed passengers to continue travelling in the rear cab despite it being out of bounds to the public?". There isn't an industry rule that forbids being late, IMO that's the difference. I respect your opinion and the inconvenience that troubleshooting the nuisance brings, but for peace of mind I'd prefer that more was done to prevent it.