The highland mainlane would be a great addition to tsw as its 118 miles long running from perth to Inverness and would utilise the class 170 in the early scotrail livery when they were new to the line in 2001
118 miles is a tad too long. The longest route (Kassel - Wurzburg) is 100 something, but its mostly bridges and tunnels with only 3 stations.
More likely to get Aviemore to Inverness! Anyhow regardless of length this is a route that cries out to be set in classic BR loco hauled days. Pairs of Class 26’s double heading 15 coach overnight services, with Class 40’s and the occasional 47 also putting in an appearance. It would just not be the same experience from a modern Sprinter unit with more than enough power in reserve.
It would still be quite the experience with a 47 I7C, 158 or 170 flying through the Grand Scottish countryside.
Sadly the one iconic thing we will never experience in TSW is a pair of Black Fives (or BR Standard Fives) heading 16 ex LMS P3 coaches over Slochd or Druimachdar Summits.
Nor Caledonian locomotives heading up Glen ogle, over the viaduct on the marvellous stretch of the Callandar and Oban Railway
Aviemore to Inverness is around 40 minutes which could open up this route into a larger Highlands-based DLC with 2 additional routes. 1. Inverness - Aviemore = 37 minutes - Carrbridge and Aviemore 2. Inverness - Ardgay = 1 hour 29 minutes - Beauly, Muir of Ord, Conon bridge, Dingwall, Alness, Invergordon, Fearn, Tain and Ardgay 3. Inverness - Elgin = 46 minutes - Inverness Airport, Nairn, Forres and Elgin
Have said previously a small network based around Inverness might be the best way to approach this area rather than a long linear route that would start at Inverness but end in the middle of nowhere due to development time constraints.