The world is too rigid like this. It's a mural, in a league completely different from marking gang territory. The artist(s) probably didn't have specific permission to paint it, but if whoever owns that boxcar has no problem with it, then there's no problem. Painting something this large for anyone who looks to see is no more a crime than street performances. Besides, it's not like it's on a department store's wall or something. It's on what was once an eyesore, probably already tagged at least once.