Have a look a Dalmunzie by vern
Afraid my still not yet finished version of the Ffestiniog and Welsh Highland is current time period, although when it's finished I can revert it to an older period, thinking ahead I made sure the dem covers the bits that were left behind just have to delete a few hundred thousand trees and realign the track in a few places and all the buildings and roads that appeared after the 1940's
We seem to be a bit short on actual UK Narrow Gauge routes of any gauge that are actually finished and resemble anything like prototype, there mainly seems to be a collection if part finished bits!
Someone needs to do a proper version of the Lynton and Barnstaple, or I'll have to do it myself, if I ever finish the FR/WHR over 4 years working on it now.