Apparently the GP15 model has the same problems that the GP7 did, odd textures. The fix remains the same:
navigate to 'Assets\DRB_SimStuff\NA\RailVehicles\Diesel\GP-15-1 CSX 1505\Engine\Textures\'
find 'GP15_Body_nm.TgPcDx'
copy to your desktop
rename to 'GP15_Body_nmc.TgPcDx'
copy over the file of that name in the original folder
This will keep the original MP skin from showing at times.