This is a repaint of DTG GP40-2Pack01 (Train Simulator: GP40-2 Loco Pack Add-On) into three Reading GP40-2 units numbered 3671, 3672 and 3673 like the real ones.
In order for this model to work, you need to already have DTG GP40-2Pack01.

Installation requires to edit a geopcdx file, for this it is highly recommended to use Mike Simpson's excellent TS-Tools utility.

Installation
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1- Open the zip archive, find the Assets folder, drag it to your Railworks folder. Answer "Yes" to the prompt about merging the Assets folders.
2- Drill down to DTG\GP40-2Pack01\RailVehicles\Diesel\GP40-2\NS\Engine folder. If the GP40-2Pack01 folder is empty except for a file named "GP40-2Pack01Assets.ap", open this file (it's just a zip archive) and continue drilling down until you find the folder.
3- Copy the GP40-2_NS.GeoPcDx file to your Assets\DTG\GP40-2Pack01\RailVehicles\Diesel\GP40-2\Reading\Engine, Reading2\Engine and Reading3\Engine folders.
4- Now to see your engine from inside the cab you need to copy and edit the following GeoPcDx file.
  - Go to DTG/GP40-2Pack01/RailVehicles/CabView folder. Copy EXTERIOR_NS.GeoPcDx and paste it at the same location. Rename it to "EXTERIOR_Reading.GeoPcDx"  and open it with TS-Tools. The file should be displayed in the "RW_Tools Editor" window.
  - Now open the editor's Search window (Ctrl-F), type "\GP40-2\ns\" in the upper field, type "\GP40-2\Reading\" in the lower window, and hit the "Replace All" button. This will change all twelve occurences of this string. Close the Search window.
  - Finally save your changes (Ctrl-S) and close TS-Tools.
5- Do the same thing with the Reading2 and Reading3 if you want to drive those units and see the textures from inside the cab.     
 
To use in a scenario
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In the scenario editor, enable content from DTG/GP40-2Pack01.
Now you should see the GP40-1 Reading Units along with other units from the DTG DLC.
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I want to thank Chacal for his precious help!