I made this today, here is how I do it. I haven't gotten the hang of getting the textures in yet, but it will come sooner or later. The DEM acquisition is, unfortunately, USA-Centric, but after one has obtained their particular elevation products, the rest of the process is still quite applicable. I am open to constructive criticism (suggesting ways it could be better). I will ignore bashing and not even think about any criticism that does not include helpful information on how to make it better. I hope it helps those of you who need it.
This is a PDF tutorial, made in OpenOffice. Took about 7 hours, including route creation, typing, editing, eating, the other thing, proofreading, etc...
For those worldly people who will bypass the US-oriented acquisition, stay away from 30-meter (1 arc-second) dems. 10-meter (1/3 arc-second) is ok, but no more than ok. 1/9 arc-second dems (3-meter) are perfect for the resolution of Trainz terrain. There is no reason 30-meter dems are any easier for a beginner than 3-meter dems. I made my first TranzDEM route with 1-meter files; they look no better than 3-meter dems until we finally get HD terrain in Trainz.
Also for my worldly friends, if you are unable to find dems that work from your country, try The National Map method in my tutorial just for the fun of it, to get an idea of how the rest works.
Cheers.
https://drive.google.com/file/d/1fqV8jQFhyR6D7BhlvdyH4F6JJjvLVmSM/view?usp=sharing