Hello, welcome to the forum.
I'm not too familiar with Scotland or even North of England but when I last visited Scotland in 2012, I remember that the A86 road into Newtonmore (just south of Aviemore) runs alongside the railway.
Wherever you end up going, check out
www.realtimetrains.co.uk to get a good idea of what you can expect to see - use the detailed search and you just have to enter the location and date.
Hope this helps.
Tony