I would like to start this post out by stating that I am not a graffitti artist, but I am familiar with the 'scene' that is graffiti. I would also like to state that graffiti comes in a couple of different forms. There is the occasional "Big Mikey and Little D" stuff that small children put in wet concrete, there is the 'tagging' where someone puts their alias in/on anything. Often times you seen tags scratched into the back windows of SEPTA buses. If you walk around Center City you can look at the railings, the metal shutters on windows and the like and find little tags done in white out and marker. Then there are full on 'pieces'. Pieces are what people are used to calling graffiti, extensive full size lettering done in multiple colours on walls and the like. I am assuming that we are talking about Pieces here.
For those paying attention, there are perhaps about 50-100 'hard core' writers in Philadelphia. These people have perfected the art (and yes it is art) of writing and piecing. Every verticle surface is a chance for them to spread their art, and this includes trains. So far the only SEPTA train I've seen that has been pieced is a car that is sitting in Overbrook (Sil II or III I think). Otherwise SEPTA vehicles have faired well (or SEPTA cleans them quickly). This cannot be said for the freight railroads. A quick glance at any frieght train usually weilds at least one or two cars that have been hit.
Now that you know more about the problem, what is the solution? Here is where it gets hard. You can catch the small time kids (yeah, they are usually kids) that paint every once in a while. The 50-100 hard core graffiti artists are another story. The one tag or piece that you see is nothing compared to the other thousand that are all over the city. These people are professional artists, and with the art comes the crime of trespassing and actually painting, which they have perfect as well.
The point is to illustrate that these people do not go out with the thought 'Hey I'll disrespect this train by painting it', it is more along the lines of 'This train goes a lot of places, and people will see my art work when it is going/sitting at the station/sitting in the yard'. The graffiti 'problem' that SEPTA has now is not nearly as bad as what it had years ago (70's/80's), what New York has had then and now, and what goes on in Europe. So I suppose SEPTA is doing something right.