In coach, you can use the bathroom facilities to wipe down with towels - but, I wouldn't use their water supply. I would use bottled water.
With the sleeper cars in First Class, depending on the size, you have a stand-up shower, a toilet stand-up shower [tricky to use], or there is a separate full bathroom w/full shower for use by "Firsties" [but you have to share the bathroom with others] ...
I like trains, and isn’t there lay overs where one can take a break anymore between trains...?