In my mind it's an engineering issue. Unlike a traditional flushing toilet, on the ground, you don't want to be carrying many litres of water in a header tank to push everything down round the bend, because that's weight you need to carry aloft for no other good reason (if it doesn't all get used, it's a waste, if it does all get used then what are you doing with the rest
of the waste?).
So use a pressure-tank in reverse to pull
the deposit through. You can 'recharge' the vacuum, either directly from the outside but more likely just pumped out through a valve more like an airlock flusher (without the need/desire to bring the air and its newly-collected volatiles back into the cabin), but the toilet bowl is never connected directly
to the outside, and the moved movements and liquids are settled into a collection tank below the vacuum 'container' that shouldn't have anything other than gases (maybe a bit of spatter) pass into it.
(Space-loos cannot be so wasteful of atmosphere, nor rely upon gravity to seperate the air from the rest. Centripetal separators and substantial air-treatment filters are probably just the half of it.)
Though trains have
, and do, still work with a practically Thomas Crapper-based approach to provisioning passenger's 'pleasures', there'd be many advantages to taking an plane-spec system and installing it in a train (an inter-city service, with many passengers needing to go a long way, with a long way to 'go' ) to reduce the need to refill quite so large water tanks and no longer dump the waste out onto tracks (which I'm not sure happens much/at all now, on our fairly civilised system) or using flush-water expanded holding tanks rather than ones large enough to deal with air-flushed deposits.
But this is just a thought experiment. I've no links to the industries of rail, air or mobile waste management from space-loo to porta-loo.Humorous news: http://www.bbc.co.uk/news/newsbeat-44005284
(I got 9/10, rather proudly, only failing with the baltic states question where I dithered the wrong way between the right answer and the one I chose.)