Faslane Harbour was built during the Second World War as an additional docking facility to counter the possible destruction of Londons docks as well as docks of other cities. The port at Cairnryan was constructed at the same time to provide further capacity.
The line was built to connect Faslane to the West Highland Railway (owned by the London and North Eastern Railway at the time) at Faslane Junction. The branch ped from the West Highland Line as a double track before ending at a reversing spur at the west end of the Faslane Harbour. From here a series of sidings ran to the Faslane pierheads. The railway was built to European standards of track and signalling to give troops practise before the D-Day invasions. Due to the single track nature of the West Highland Railway the loop at Garelochhead was lengthed and a small marshalling yard installed at Dalmore (on the Glasgow, Dumbarton and Helensburgh Railway) to the east of Craigendoran Junction.