klioli wrote:
I would prefer to start with a small section first
Absolutely.
Due to the static mapping of UTM coordinates to Trainz baseboards by TransDEM, you can seamlessly merge in Trainz Surveyor. You should use the same DEM data source for your modules and stay in the same UTM zone for this.
If your route crosses a UTM zone border (every 6th meridian, e.g. 12° east, Halle or Regensburg in Germany), you should define your primary UTM zone, the one with the bigger part of your route, and let TransDEM convert all geo data from the 2nd UTM zone to the primary one. It's more or less automatic if you load geo data from the primary UTM zone first.