If you don't own a train company then you go and paint on one instead... it all comes from that thing at school when you had to have name tags in the back of something... that makes it belong to you. You can own half the city by scribbling your name over it.
BanksyTraditional graffiti writers have a bunch of rules they like to stick to, and good luck to them, but I didn't become a graffiti artist so I could have somebody else tell me what to do. If you're the type who gets sentimental about people scribbling over your stuff, I suggest graffiti is probably not the right hobby for you.
Banksy