After a very long time here is another status update.
I stopped adding more detail to the train and instead turned to the tedious, time-consuming task of UV mapping.
Progress is slow but steady, and almost all of the carbody has a space assigned on the texture.
I had planned to make more progress, especially during lockdown, but my computer died and I first had to wait until it was fixed.

Here a current screenshot in Shape Viewer:
(click to enlarge)