The youtube clip is part of a much longer DVD called "Railways at War : The First World War – I bought the DVD which was well worth the money. Briefly, Andrew, the steam locos, coal-fired Hunslet and Baldwin 4-6-0T's and ALCO 2-6-2T's like 'Mountaineer generally hauled munitions and supplies from the rail head. Closer to the front the Simplex 'tin turtles' and Baldwin tractors took over but this would always be on steel rails. Occasionally wagons would be horse-drawn so that MAY have been over temporary wooden rails.
There are several excellent books on the subject such as 'Narrow Gauge at War vols 1&2' (Plateway Press), 'Narrow Gauge to No Man's Land' (Benchmark) mainly the U.S. sections, and the new volume from Roy Link 'WDLR Album' containing original photos and excellent drawings. Apedale Heritage Centre have a selection of original WW1 locos and rolling stock. Hope this helps – my maternal grandfather was at Ypres – in the Argyll & Sutherland Highlanders and he didn't talk about it much either. Ray.