I don't approve or disapprove of tagging rail cars. If the rail companies actually saw it as damage, they would put the necessary resources into stopping it. They obviously care very little.
For me, I get to shoot it almost every day as a train comes right past my home here. I've got a gallery with 1600 rail pictures, almost all graffiti.
The guys running the local train got so curious eventually they stopped the train one day and asked me why I was taking the pictures all the time. When I told them it was for the graffiti they said, "Oh, well we'll have to get you some better cars."