Trains, as far as I'm aware, just dump human waste on to their own tracks. There are often warnings like:
DO NOT USE the toilet while the train is stopped at the station voiced through the PAs or signed up on the walls. The Daily Mail, in 2014 had an article mentioning:
Train companies are to be ordered to adapt their trains to stop dumping sewage on the tracks.
I always kind of assumed that commercial airliners did the same, but I've never heard any warnings about using the restrooms aside from when the seat-belt sign is on.
- How do commercial airliners take care of human waste?