How is an engineer able to "answer the call of nature" without releasing the dead-man's brake? Would somebody else have to take his position until he returned?
Most of the time... well... you just tend not to use the restroom in the train.
I will explain why as succinctly as possible.
That is the typical restroom in a typical CSX locomotive.
Nine times out of ten the engineer simply stops the train near a gas station or a train station and uses the toilet there. (I've seen the crew of the CSX garbage train from New York stop and use the restroom at Alexandria Union Station every once in a while when I'm waiting for the train home from school.)