Based in Inverness, the company was formed by merger in 1865, absorbing over 249 miles (401 km) of line.
It continued to expand, reaching Wick and Thurso in the north and Kyle of Lochalsh in the west, eventually serving the counties of Caithness, Sutherland, Ross & Cromarty, Inverness, Perth, Nairn, Moray and Banff.
[1] In 1923, the company passed on approximately 494 miles (795 km) of line as it became part of the London, Midland and Scottish Railway.
As well as the Kyle of Lochalsh Line that branches at Dingwell, the Black Isle Branch left at Muir of Ord for Fortrose, the Dornoch Light Railway left at The Mound for Dornoch and the Wick & Lybster Light Railway ran from Wick to Lybster.
Between 1885 and 1946 a branch left the line at Fodderty Junction, 138 miles 56 chains (223.2 km) from Perth.