In many places in Europe there is a de-facto truce between graffiti artist and the local governments: You can paint/tag some areas.

Typically noise barriers are such, while trains themselves or stations are not.

Harder persecution both by authorities and self-policing in the subcult community after some prominents were persecuted and agreed with to help normalize the situation has led to the current status quo, which for example in Hungary has normalized the situation pretty much, public transport and stations are generally not vandalized, and some larger well exposed areas were designated as local "legal walls".

I guess the panopticon (cameras becoming cheap and ubiquitous) also helped