PC: What's the "A" for?

[Jerry Jordak <jer@xxxxxxxxxxxxx>]

Hi everyone,

A friend of mine was asking me about a marking on a Third Rail Graphics 
boxcar he bought. On the left side of the car below the "Penn Central" 
and above the reporting marks is a yellow circle with an "A" in it, like 
on this car:

http://www.rr-fallenflags.org/pc/pc160269ajs.jpg

Anyone know what that marking was for?

Thanks,
-JWJ