we have 72 pax per coach on journeys upto 3 days. nobody has this time x density problem. I suspect the long haul trains from china east coast to interior and the trans siberian trains have far fewer pax per coach and the chinese trains are anyways increasing speed by leaps and bounds - they have two levels of HSR already one goes 200k and other around 350k.
I see no option but storage toilet or that retention system thing but which will foul the tracks royally within city limits as the train picks up some speed. if the poo poo is released at a higher speed as the train goes further away, the air pressure under the wagon will disperse it far and wide spraying it like wet cement all under the carriage and the wheels leaving a smelly crusty mess for the people who work on the wheels, suspension and batteries back at GHQ by walking down that recessed tunnel on the ground between the tracks at the depot.
imo go for storage, and make sure some automated vacuum sealed cleaning system is used at major halts to make sure the good stuff doesnt backflow and created "issues" sometimes seen in aircraft lol
some ppl actually prefer to use it at stations as the rocking and howling wind scares older infirm people, women and children
during my college days sometimes the end of the bogies would lurch almost 6 inchs up and 12 inches laterally ... trying to squat down and do it was quite a feat I realize now.