In case anyone is interested, I finally managed to get a route finished and uploaded to the N3V DLS.
It's called "Highland Link" and is a "factional" (real locations, fictional railway) concept of a railway linking Corrour on the West Highland with Dalwhinnie on the Highland Main Line, along the popular walking route.
Running distance just over 30 miles.
It's for Trainz A New Era (TANE) only and on the DLS as KUID2:30024:20077:1 - uses all built in or DLS assets, no external third party stuff required. You will need to set up your own sessions as trying to set up a player driveable but automatic route setting session (with timetable) in Trainz still eludes me.