Although in 1860 Inverness had a rail link to the south, the route via Aberdeen was circuitous and involved a change between two railway stations over apart. Passengers were conveyed between the termini by omnibus, paid for in the through fare and with forty five minutes being allowed for the transfer. The GNoSR refused to hold its trains to connect with trains arriving at Guild Street. The mail train would be held until the Post Office van had arrived and the mail was on board, but the station locked at the advertised departure time to prevent connecting passengers further delaying the train. A more direct long route south to Perth, bypassing Aberdeen, was planned. This left the Inverness & Aberdeen Junction at Forres on a steeply graded line, before heading south to a summit at Dava, then to Grantown and the River Spey to Kingussie, before another climb to a summit at the Pass of Druimuachdar, at the highest in Britain. The line then descended to Birnam, from Perth and linked to Perth by the Perth & Dunkeld Railway since 1856. Although a similar route had been rejected in 1845 because of the high altitudes and speed gradients, there had been sufficient advance in locomotive design to satisfy the fears of the parliamentary committee, and authority was given in 1861. A long girder bridge crossed the River Tay near Dalguise and a viaduct crossed the River Divie near Dunphail. North of Dunkeld was a long tunnel and with through the Pass of Killiecrankie. The line was initially single track with passing loops at stations, the IAJR doubling the track for between Inverness and in 1864.
