Asphalt tankers

[kdewald]

Does anybody know what the, I'm guessing 3 inch, round stickers located on the ends of asphalt tank cars are for ??  They are different colors. Blue, green, red, orange. These tankers are the only ones I see with these stickers. 

[nsdtk]

I don't know for sure. Just know that when i spot asphalt at a roofing company. There is different grades and weight of asphalt. Maybe the color coresponds to a type of asphalt.