Go to this directory
C:\Program Files (x86)\Steam\steamapps\common\RailWorks\Assets\HIS\WMRetroPack01
unzipthe file: WMRetroPack01Assets.ap
Copy the contens of the folder: WMRetroPack01Assets
paste them here: C:\Program Files (x86)\Steam\steamapps\common\RailWorks\Assets\HIS\WMRetroPack01
navigate to this directory: C:\Program Files (x86)\Steam\steamapps\common\RailWorks\Assets\HIS\WMRetroPack01\RailVehicles\Diesel\GP35\Circus  
paste the file & Click yes 
you're done Nejoy 