This article in El País Semanal focuses on graffiti artists who paint metro and train coaches, a practice they have been performing for decades, but that is getting more media interests in the past months because of some isolated cases of violent behaviour. According to official data, of about 3500 cases of graffiti incidents registered in Catalonia last year, only 7 were reported to be violent. 

The vast majority of graffiti artists, however, are far from violent and value anonymity: they enter, paint and go. If they are discovered, they run. Respect among graffiti artists is another reason to look for the most hidden, less accesible coaches: the hardest the access, the highest respect and admiration they receive from other "writers", as they call themselves, even though they protect their anonymity as much as they can. José Sánchez, Scientific Coordinator of TRANSGANG project, states in this report that graffiti has more to do with the search of adrenaline and excitement that it does with vandalism, there is an hedonistic side to it, reflect of the society we live in. 

https://elpais.com/elpais/2019/02/11/eps/1549881701_341660.html