North-western Scotland is weird, and I say that living there. We have some good rail routes, but there are limits. The existing lines usually use two carriages and aren't super frequent (and take a while, 5.5hrs from Glasgow to Mallaig on the West Highland line) and outside the height of summer its often very empty. Wonderful life line, but there are limits given our low population density spread amongst the mountains (plus there's a lot of public transport here that aren't trains, like the ferries that connect the islands and peninsula communities). It's odd. Probably could do with a line from Fort William to Inverness, but that's the only real demand left. Most of the people live on the east coast, which is served by Edinburgh-Dundee-Aberdeen-Inverness line, so it really just needs a line for people on the west coast to easily access the capital of the Highlands.

The Borders are also pretty sparse, again due to mountains and population, but you can see the Central Belt of Scotland and the major cities of Northern England have quite a lot of rail.