For a public service (and this is public transport remember), you should look after the public and the cost of maintaining toilets on a train is a minimal cost across a large organisation - you could literally add cents to a ticket price and this would easily cover any cost.
It's gonna cost a sharkload more than a few cents per ticket to replace all the brand new carriages with ones that contain toilets.
How often do people actually piss themselves on trains?
Early hours of Saturday and Sunday morning, probably about every ten minutes! ;)