I have just released a little backdating kit for the Stevens Pass Route DLC.
What this does is add depots, station signs, train order signals, platform clutter and station markers to the route. This was all done in the scenario editor only, so the installed route remains completely unchanged.
You need a bit of knowledge of the inner workings of the scenarios, crucially their folder structure, and of course you must know how to build and edit your own scenarios if you want to use this.
Detailed instructions are available in the manual.
You can download the backdating kit on my website.
Below are a few screenshots of how it looks like in-game.