What new Trainz? The next version is a single route, Settle & Carlisle, no other new version planned as far as I know.
Now if that was a narrow gauge line.......(speaking as a Trustee of the Welsh Highland Railway Porthmadog with about a mile of running line at present, though soon increasing to 22, FR permitting..)
That said, you would be hard pushed to improve on the narrow gauge content already available from members, such as Murchison (and still coming, see the WWI thread in the Freeware forum)-there's quite a lot available already.