This is how I remember it, two verses:

If your train is in the station please refrain from urination
have respect for railway property
If you want to pass some water go and ask a railway porter
he'll direct you to a lavatory

If your train is stationary and you want to go quite heavy
have respect for railway property,
from the carriage you must wander and a penny you must squander
to relieve yourself with might and main