Just a quick message to note that Building Tools is already in version 1.0.6 and it's a fantastic tool to create buildings fast even with little or no 3D design experience (like me!). https://ranjian0.github.io/building_tools/ Hope you find it useful!
Something that I completely missed in my post earlier this year: Blenderkit! They have a free plan with plenty of stuff in there. For instance materials (e.g. facades, floors, tiles) and objects (e.g. architectural elements, exterior objects). As of writing there are 1400 models and 1896 materials! And best of all: You can access all of them directly from Blender Also a very good resource for Blender materials I was just made aware of, is Julio Sillet's (on gumroad).
Tracks are full of hard surface models, so the little but awesome JMESH Blender addon probably helps alot! You can find the documentation here and download it on github A preview of the awesome features you can check tutorials and videos on youtube, like this one.
Not strictly track modding related, but again a nice tip for hard surface modeling is shown in this short video
Here is something for those that like to model after photo references and need a tool to match the perspective as best as possible: The Perspective Plotter It's an add-on for Blender that costs $15 (w/o tax). Alternatively, there is another software with the same purpose which is completely free and open source: fspy The result can be used in any 3D software, however, it may work best with Blender as there is an official import addon-on for it.