It's fortunate for Scotlands rural lines to be existing in this new world of green targets and net zero policies. Lines that would most probably get closed are relatively safe. Any government attempting to close them would be commiting political suicide.
Summer patronage does go some way to keeping the costs down but the subsidy per passenger year round is high. It would be unthinkable to close these Highland lines but as far as locals would be concerned, I don't think they'd be missed too much.
I do think that a regular steam service from Inverness to Kyle would bring in a few tourists.
Depends which line as to whether it’s used much by locals. Locals from Kyle, Plockton, Lochcarron, south Skye etc definitely do use the Kyle line (my local line) for shopping trips etc to Inverness, particularly as it’s cheaper and more comfortable than the Citylink bus, which does not serve Plockton or the Lochcarron area at all. In the summer months they probably are very much outnumbered by tourists but there are still enough users to justify four trains each way per day Monday to Saturday all year round. The Sunday trains to and from Kyle seem just as busy in winter as in summer. It’s very rare that I travel on a Kyle line train which is overcrowded but equally rare to travel on a quiet one, at any time of year.
I rarely go to Oban by train but when I did use the service, on a rare sunny Saturday in early November 2023, there were quite a few locals travelling from Taynuilt and Connel into Oban for shopping or to visit the pub. The train was mostly day trippers or tourists until Taynuilt but was still remarkably busy for the time of year. I am aware that the Oban line has school traffic from Dalmally to Oban High School, some kids from Plockton High School also use the Kyle train to return home from school.
The Far North Line north of Tain is a completely different story - I doubt anyone in Wick/Helmsdale/Golspie would use the trains out of choice because the route is so indirect. Winter loadings for this route must be very low.
I’m almost never on the West Highland Line north of Crianlarich except on the sleeper so can’t comment about usage of those services. I cannot imagine many Fort William folk preferring it over the bus to Glasgow because of the indirect route and much longer journey time.