How are they different if any from just the Gp9?
Different railroads used different terminology to describe an upgraded GP9. The most common upgrade is to replace the 567 power assemblies with 645 power assemblies, this increases each cylinder's displacement as used in GP38s. The reason for doing this is to reduce the number of different parts stocked. Other common upgrades include replacing the generator with a D32 DC generator and replacing or having the traction motors rebuilt to D77 standards.
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