The Pelican
Member
I think substantial speed upgrades might actually be easier on the railway than the road in this era of national parks- you just need a single track tunnel which cuts off one of the twisty mountain sections. This chart- https://www.railwaydata.co.uk/linefiles/route/?ELR=HGL2 suggests that if you could tunnel between miles 45 and 60- a tunnel of less than 15 miles- (perhaps closing dalwhinnie, or putting it on a branch line from one end) you could substanially reduce the height trains had to reach. Obviously needs a view from above as well, to work out where the slow bits are. The work on the Bergen line in Norway has done lots of bypassing and closing of the slowest parts.
