I'm guessing that if you turn up as a pair or more then one stays behind and the rest go off to a local Mcdonalds, hotels etc or similar, that have toilets and sinks to wash in.
I know that some years ago some of the big stations in london had pay to use showers, not sure if thats still the case.
if you are by yourself then i suppose if you get friendly with the people in front of and or behind you, they would be asked to look after your place. whilst one goes off in search of cleaning and food places