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

I think if it was put to a vote, most people would be against graffiti.

These guys know the city in and out. Only in cases where you already expect them, you could be able to catch them. Also, police always has to prove you did it, and not some other member of the group.

https://youtu.be/Y9Lm0dkkVAw?si=NDHBYtciFPDhp78I

Almost everywhere, but it's fairly low priority and impossible to prosecute without Orwellian levels of surveillance.

We already have those levels of surveillance. It's just an unwillingness to do anything.

Honestly, it's just paint, it has no direct victims. With most police departments understaffed and underfunded (at least where I live, Portugal), I would rather they focused on more serious problems.

I'm pretty certain it's an offense in most developed countries.