There is an article on the Lochaber Railway in Railway Magazine Apr 1955 (p241 on) which states the line from Loch Treig to Loch Laggan was built in 1931 for the construction of a tunnel down from Loch Laggan to Loch Treig. This implies it did not last long after that ?
The RWM article on the original construction of this section is in the Jul 1931 edition (p73).
In that era it would have been quite common for civil engineering contractors to use temporary narrow gauge railways for moving materials to a site, and within a site, particularly in remote areas with little or no road access. A longer and earlier example (though standard gauge) is the fairly well known line to Talla reservoir in the Scottish lowlands:
en.wikipedia.org
The
Talla Railway was a privately constructed railway line in southern Scotland under the
Edinburgh Water Company. It was built 1895–97 to aid the construction of the Talla Reservoir, to serve the water demand of Edinburgh. The railway was about eight miles (13 km) long, running south from a connection with the Caledonian Railway's Peebles branch at Broughton. A private passenger service was operated for workmen on the reservoir construction.
It's also quite possible that there would have been temporary track laid within the tunnel for construction purposes. In the mid 1970s I worked on construction of a dam which incorporated a tunnel, and we had temporary 2' gauge track (in panels aka Jubilee track) laid through it, with wagons hauled by small battery electric locos. My then employer owned the equipment, which had been used on a number of previous contracts, although I think that was probably its last.
So the contractor(s) for the Laggan dam and tunnel would probably have been very familiar with temporary railways, and might well have had the necessary equipment available already.
