Why is sleeping at a train station fine? (Unless you're talking about infrequent long distance trains).

Trains are transportation infrastructure. Drivers don't have to put up with people setting up a queen bed in the middle of their lane on the highway. If a country doesn't protect its public transport infrastructure, then the rich and the middle class stop taking the train, the poor have to put up with things, and the mentally ill get an overpriced and noisy mobile homeless shelter. One that costs more and helps less than crisis accommodation.