Call me a hard ass but in my 33 years career at NYCT, I'd like to say I've heard and seen it all.
Sure it sucks to see it tagged before it has entered service, but once any piece of equipment is placed on NYCT tracks, all bets are off as far as vandalism is concerned. Even if an employee saw the deed being done, all he can do is report it, he will not risk his personal safety in chasing or apprehending the culprits. Police will be called. But guess what? By the time police show up, the culprits are long gone. Plus even if the culprits are still there, the cops will not go on the tracks unless power is off.