Simple scenario; what if you want to use the loo while waiting for a (probably delayed) train?
I think what you're supposed to do is go outside, lock it to one of Euston's finest double-decker bike parking spaces using your touring-grade cable lock, then carry your four panniers and any loose accessories into the toilet cubicle which barely has room for a sanitary bin. And hope the bike's still there when you shuffle back.
In more civilised places
[1] I tend to ask someone working in the cafe or the ticket office or whatever if they can keep an eye on my bike while I nip to the loo. It's one of those things where being in fully loaded touring mode helps, people generally being more predisposed to being helpful to a traveller on a mission rather than just a bloodycyclist.
Take it into the disabled loo with you?
I've done this too, but only at quiet times or when [TMI] means I'd be legitimately using the accessible toilet anyway.
Or just hold it and run the risk of train toilets later...
[1] Not Edinburgh Waverley.