That's what I thought.
Unfortunately, out on the island Lidingo, it IS the local teens with nothing better to do. You can tell because as soon as the guys paint the cute little train stations, the next day they're all marked up. (And then they start painting again).
Bet that could be nipped in the bud if they didn't allow spray cans sold to anybody under 30.
Also, Tunnelbana trains coming up from the Norsborg area (south end of the red line) always seem to be in worse shape than the others...???