Sketch-up models work for the most part, but they are not recommended due to the models being too high poly, individual textures, and no LOD. This has always been the case with these models imported into T:ANE and even TS12 for that matter. The models used in moderation work fine, as I've created and still use them myself, but I wouldn't populate a whole town or city with them due to their construction.
There are other programs available for modeling including Blender, which are even better because of their direct export into the FBX format. FBX is a cross-platform, multi-program file format, which will export a Blender, or some other program's files, into the Trainz mesh format. With FBX the user can use Blender, 3ds Max, or some other program as long as that program supports the FBX plug-in.
I agree that there is a bit of a learning curve associated with these programs, but that's the nature of the beat with them. I admit too that I am way out of practice considering I haven't modeled anything since maybe the mid-1990s when I used 3ds R4 for DOS, and early Max versions so hats off many, many times to those that produce those wonderful assets which I am too so grateful for.
Anyway, what is wrong with your models that aren't working, but worked in previous versions? There's a number of reasons why, but if you show us the error messages, perhaps we can help you fix the problem. The no-go issues with assets are usually related to scripts and LOD targets and the increased requirements surrounding LOD due to the push for better performance.
