Hi,
That was the right file to look at. Internet Explorer is probably set as the default program to handle .xml files in your Windows settings, you can change that any time if it bothers you (or just right click on the file and select "open with" and pick the program of your choice, like say Notepad++ if you want to edit the file).
Those four entries in the <Skies> section tell us that your route is set up to use the seasons from the Academy route. See those four near-identical blocks called <SpringSkyBlueprint>, etc. -- inside them are the references for where the game will look for the files. In this case, provider DTG, product Academy, and then the file path. So in Explorer speak, this translates to Assets\DTG\Academy\TimeOfDay\Spring.xml, etc.
With that in mind, check that your route has the asset filter ticked for the Academy assets. And in the probably highly unlikely event that those Academy assets got deleted or damaged, a Steam verify might help.
Alternatively, you could also edit the RouteProperties.xml to link to a different set of TimeOfDay blueprints if you don't want to use the Academy assets (and thus have to keep their asset filter enabled forever). Simply open the .xml file in a text editor of your choice (I think even WIndows' own built-in one might do the trick) and replace the references as needed. For example, you could replace all four calls to "DTG" and "Academy" with calls to "Kuju" and "RailSimulator" in case that's an asset set which you are using on your route.
Create a backup copy of the .xml before editing it.
Hope this helps.
Cheers,
Michael