Once again, digging up a long dead thread, but I want to give my support for Sketchup and rubyTMX
Like most things when you say "sketchUp" it gives a bad taste in the mouth, but that is normally due to people not knowing how to use it. The game Kerbal Space Program used Sketchup to create its assets.
But if you are smart, if you apply "real" game design logic to this, you can prosper. Make a single texture map (square) and then use that one texture to paint the whole asset, using the "texture>position" option in the right click menu. and then create multiple versions of the same asset with your own manual LOD's and then arrange the exported meshes into one folder using this method:
https://online.ts2009.com/mediaWiki/index.php/HowTo/Make_your_first_Trainz_Asset
in a very very basic test i have done with creating my own manual lod files, i created a commie block that was as "high" as 426 ploys, and as low as 12.
https://imgur.com/a/IhgTW9G
It is very easy to get good quality and usable models from SketchUp. I did another "test" where i picked 50 assets at random, and not one of them had any sort of LOD.
So yes, you take a model from sketchup warehouse and want to upload it to trainz, it might be poly heavy. RubyTMX doesn't do LOD's by default. and if you aren't "clever" about creating your file you may have issues, the example of a cube with a cube protruding. if you build them as a merged entity, there will be more polys, than if they just overlap and never digitally interact with each other. If you are an engineer this will be screaming at you, but for videogame assets, it is what is expected.
so i think every criticism put against sketchUp can be worked around, but that is just it, a workaround. Blender can do this for you, but I have never got on with Blender. of course Blender also has a much better way of handling more advanced textures natively.