Most trains have toilets on them. The only ones that don't tend to be the local 2-3 carriage bus on tracks style ones.

Larger stations normally have toilets - but they're not necessarily in working order, and the odds are they're not on the "platform side" of the ticket barriers, so maybe a 5 or more minute walk to get to.

Quite often the train is only in these terminus stations for 5 minutes.

Smaller stations in-between are normally just two platforms and a bridge to get to the other side.

even the ones that do normally have them locked for whatever reason now. they do normally have toilets on the actual train itself though