Go to this directory
C:\Program Files (x86)\Steam\steamapps\common\RailWorks\Assets\Reppo\Gp20PackWP
unzipthe file: Gp20PackWPAssets.ap
Copy the contens of the folder: Gp20PackWPAssets
paste them here: C:\Program Files (x86)\Steam\steamapps\common\RailWorks\Assets\Reppo\Gp20PackWP
navigate to this directory: C:\Program Files (x86)\Steam\steamapps\common\RailWorks\Assets\Reppo\Gp20PackWP\RailVehicles\Diesel\Gp20WP  
paste the file & Click yes 
you're done Nejoy 