Trainman,
There is a lot of external diiferences (besides the number of handrail stanctions) between the GP59 & GP60. If you don't already have a copy, get a hold of the March 1993 issue of Railroad Model Craftsman. In it you will find drawings of both the GP60 & GP59.
In a nutshell the GP59 has a shorter engine compartment, 6 doors vrs 8, and the inertial air filter inlets and traction motor blower enclosure are located 21" further back from the cab.
Dan