It was planned/proposed in the early 20th century.
I can vaguely see the alignment, though the gradients up Glen Shiel would be decidedly alarming - something like nine miles of 1 in 47 - as well as a challenging coastal section on the north shore of Loch Duich.
One of my idle fantasies is for the Duke of Sutherland to discover narrow gauge earlier, allowing for more lines to be built by the Highland Railway - he openly stated that he regretted that this wasn't the case. On a quick estimate,
all the seriously proposed Highland Railway branches in that part of the world could have been built in 3' 6" gauge by the same capital that paid for the network actually built on 4' 8.5" gauge.
Per my understanding, that extension was planned and much wished for by the North British Railway; but the Highland Railway, even more desirous of not having this competition to contend with, exerted all the pressure and influence they possibly could to prevent a line Fort Augustus -- Inverness from coming into being.
Oddly enough, this lead to the situation where the 'branch' at Spean Bridge was built to higher standards than the 'main line' to Fort William - the West Highland was basically designed as a long, straggling single-track branch with passing loops, whilst the Invergarry & Fort Augustus was designed to be a competing main line to Inverness. Presumably had it ever come to fruition in the way the promoters hoped there would have been improvements south of Spean Bridge.
I think it depends very much on your definition of 'congested'. If you've not driven the the roads in the south east of Ingerland, I guess you're not likely to know that the only road in Perthshire that ever actually gets 'congested' in real terms is around Broxden in the rush hour.
You don't even need to get to England - I was cautioned against driving from Southampton to Bournemouth on the grounds of the congestion being so bad. It turned out to be non-existent compared to my Central Scotland commute.