Although the WHL looks fantastic on TS2015, I have had so many other graphical issues in other routes that I have been maintaining a valid copy of TS2014 and quite regularly using that instead. So as an exercise I thought I would have a go at getting the West Highland Line installed and running on my separate copy of TS2014.
This has been reasonably successful. The main issue is removing ALL references to the new SilverLining weather system. After that the route and scenarios seem to run correctly, including the scripting of the new RETB Token system.

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If you would like to have a go at this you will need a separate executable copy of TS2014. I use only a single instance of Steam and if I want to run TS2014 I put Steam into 'OffLine' mode, and then run the railworks.exe program from within the TS2014 directory.
The following instructions assume some knowledge of RW_Tools and 7-Zip etc.
- 1 ) From TS2015 copy the folder "RailWorks\Content\Routes\fdb37c6f-e125-4b1e-a68c-157a378ca70d" to the TS2014 "RailWorks\Content\Routes" folder.
2 ) In the TS2014 copy of "RailWorks\Content\Routes\fdb37c6f-e125-4b1e-a68c-157a378ca70d" open the file "MainContent.ap" and drag the file "RouteProperties.xml" into the TS2014 folder "RailWorks\Content\Routes\fdb37c6f-e125-4b1e-a68c-157a378ca70d". This will allow RW_tools to 'see' the route.
3 ) From TS2015 copy the folder "RailWorks\Assets\Thomson\FortWilliamMallaig" to the TS2014 "RailWorks\Assets\Thomson" folder.
4 ) Open the TS2014 file "\RailWorks\Assets\Thomson\FortWilliamMallaig\FortWilliamMallaigAssets.ap" and drag the folder "Weather" into the TS2014 folder "RailWorks\Assets\Thomson\FortWilliamMallaig".
5 ) Using RW_Tools Clone all of the Career Scenarios, renaming them as you go. I added a suffix of (STD) on the end of the name.
6 ) Using RW_Tools delete all of the Career Scenarios.
7 ) Using RW_Tools edit each of the Standard and Tutorial scenarios. Use the "Set Weather" button and change the weather blueprint to one of the standard blueprints in "Kuju\RailSimulatorCore\Weather". These are the blueprints that start with 'RW_' eg: 'RW_Cloudy'. For most of the scenarios there are 'RW_' weather blueprints that are close to the original ones supplied by Thomson. Save the scenario, accepting any prompts about changing a Career scenario to a Standard scenario.
8 ) Open the file "\RailWorks\Assets\Thomson\FortWilliamMallaig\FortWilliamMallaigAssets.ap" using 7-Zip. (You may need to remove the Read-Only attribute first.)
9 ) Using 7-Zip navigate to the folder "Weather". Which is at the root level of the archive.
10 ) Now within 7-Zip delete all of the weather blueprint files. Ie: All the files starting with "Scottish_". There are 64 .BIN files and .TGT files. Be sure you are doing this on the TS2014 copy you have made! Answer the 7-Zip prompts about removing the files.
11 ) Finally within the TS2014 folder "\RailWorks\Assets\Thomson\FortWilliamMallaig" delete the "Weather" folder that we copied in earlier.
Anyway, that's about it. After doing this I was successfully able to run the West Highland Line and its scenarios. There are some minor graphical glitches on some distant hills, but all in all it looks OK.
This won't be for everyone, but perhaps one or two might find it useful.
Cheers.