What matters for the Wellington region?

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! ;)

It's got everything to do with basic human needs. Would you like to be a parent on a train that you rely on with a kid who needs the bathroom and has nowhere to go? Much like the UK or longer haul trains, you only need 1 bathroom per two carriages - most are usually designed to have the incorporated at a later stage and wouldn't need full replacement.