I'm waiting for someone to advise that you should take your bike into the toilet cubicle with you.
No, but it's sensible to go to the toilet, buffet etc just as the wheels start to move from the previous stop, so your stuff could not get off the train without you while in there.
OTOH, my laptop and iPad do go with me to the bog unless I've Kensington locked the laptop to the table (yes, I know you're not meant to) which would again limit the speed of getaway, though I wouldn't be surprised if some thieves carried pliers to cut those, and equally wouldn't expect it to be insured in that case.
The best advice I have read on this thread is to chain the wheel to the frame so that a thief could not just ride off on your bike.
That's a good idea, it would restrict the speed of getaway so if you saw an attempt being made, even from the other end of the coach, it would be feasible to give chase. Actual Dutch bikes (like the one I ride) have a rear wheel lock which is perfect for that purpose.