I don't currently have prototype drawings I can put my hands on, but my recollection is that the GP40 should sit about 6" taller than the GP38. The GP38 being at the 14' 6" standard of the first generation locomotives. The GP40 should match the height of the 2nd Generation SD units which I believe was between 15' and 15' 3". Also, I'm not sure when they were introduced, but I do believe the GP40 would ride on newer, modified Blomberg trucks which have a slightly different appearance than the standard Blomberg found on GP9's etc. My recollection is the major visible difference is a shock absorber located outside the frame midway between the wheels.