Without a valid World Origin (in the eyes of TransDEM) TransDEM won't add or replace UTM tiles (and vector data) to/in an existing route.
When merging two routes, the World Origin will be messed up in Surveyor. This is an error in Trainz since I started with it in 2005. Therefore TransDEM has a dedicated function to restore the World Origin in a merged route.
Unfortunately, there is an additional bug in T:ANE that resets the World Origin to the region default when you save the route. N3V says they fixed it but that wasn't really the case. Let's hope the next hotfix will solve it.
If you are using TransDEM 2.6 and still have the TransDEM info file originally created for that route and present in the route folder, TransDEM may be able to restore the World Origin from that file. At least it will try.
If unsuccessful or the file is lost, you can still restore the World Origin from scratch. It's a bit more work but doable. See here:
viewtopic.php?f=6&t=427