This discussion reminded me of what I have seen on numerous cars: not just on autoracks. Four come to mind and took place over the past 3-4 years. In order, I have seen:
Cheektowaga, NY:
An auto rack that had a good sized Chilly-Willy
A year and a half later same location, I witnessed an Avatar type character that had very "accurate" body art. Its the first time I was truely embarrased to have seen graffiti. Ironically, it was on a reefer.
Westfield, NY
Last year, the CSX local had going back to Dunkirk a head shot only of Michelangelo or Leonardo(for those who don't know these are two of the main named characters of Teenage Mutant Ninja Turtles). This was on a covered hopper.
And more recently, I saw a full image of Invader Zim(a short lived Nicktoon) on an auto rack.
All of these are perhaps the more family friendly "ideas" I have seen.
However, since this is about auto racks the most viscious I encountered was on an auto rack that had a 6-7 ft reitteration of the r-word. And sadly it was the first ledgible piece of tagging I ran across.
Take care all