Sketchup does something different. First it is not easy for a beginner to use a single texture file. Each texture file has an overhead of 200 poly equivalents. Secondly if you take two beams that are 12 polys each total 24 polys and cross them in Sketchup it breaks them up into five beams each with 12 polys for a total of 60 polys and that's how the poly count explodes. The more complex the model the higher the poly count. For something that doesn't render in real time it's not too bad but for trainz where you need to render 30 times per second it pushes the hardware. You get unseen faces as well. I don't think anyone has successfully done LOD in sketchup but I could be wrong.
If you stick with Blender 2.79b then
https://en.wikibooks.org/wiki/Trainz/Tutorial_for_Blender try the moving house tutorial. The problem with Blender is it is very rich and we only use about 5% of it so Trainz specific tutorials are best. If you look in the forum there are video tutorials as well. The exporter for trainz only works with 2.79b. For 2.8 you need to use FBX and import into it could be a minimum of TANE I'm not totally sure.
Cheerio John