Shoot, you just want to see me start digging through my piles of data, don't ya? The GP60 is longer than other contemporary EMD geeps (GP38-2/40-2, 50), by a foot or so, IIRC. Gotta find the dimensional data to post, unless someone else beats me to the punch...man, I need to get organized!
That sounded like a challenge The Contemporary Diesel Spotter's Guide happened to be on the top of my workbench so: GP38 and GP38-2 are 59'2" long with 34' truck centers GP60, GP60M and GP60B are also 59'2" long with 34' truck centers Some versions of the GP40 have longer lenghts and truck centers.
The following is a quote from Railroad Model Craftsman, 8/95, page 90, product review. Diamensionally, the GP60 is longer than the GP38-2, GP40-2 and GP50. This difference occurs at the truck centers, which are 13" farther apart than those of the GP50, and in the body which is 12" longer. The side sills also are a constant thickness unlike the GP50. To create a GP60 model, Athearn has chosen to modify thier GP50 body shell without increasing its length and use the stock GP50 underframe. This is the reason RPP makes the GP60 shell and chassis.
