The War Department Light Railways were a system of narrow gauge trench railways run by the British War Department in World War I. Light railways made an important contribution to the Allied war effort in the First World War, and were used for the supply of ammunition and stores, the transport of troops and the evacuation of the wounded.
Different track gauges were used in different parts of the world including 600mm, 760 mm (2 ft 5 15⁄16 in), 1,000 mm (3 ft 3 3⁄8 in) metre gauge and 1,050 mm (3 ft 5 11⁄32 in).
The military light railways in France were of 600 mm (1 ft 11 5⁄8 in) gauge and used a variety of steam and petrol locomotives from French, British and American builders. The Germans installed their 600 mm (1 ft 11 5⁄8 in) gauge Feldbahn system early in the war. Trench railways of the World War I western front produced the greatest concentration of minimum-gauge railway locomotives observed to date.
Britain came to the belated realisation that it needed a flexible and reliable method of supplying the front lines, bringing shells, timber, and fodder from the rear areas and their standard gauge supply points. Narrow gauge light railways were the solution.
Hundreds of locomotives were built by companies such as Hunslet, Kerr Stuart, ALCO, Davenport, Motor Rail and Baldwin to work these lines. Also, Model T Ford conversions were used. Thirty or so Companies were formed within the Royal Engineers to staff the lines. These were mostly British ex-railwaymen pressed into service, though Australian, South African and Canadian gangs served with distinction. An American unit also served under the British flag.
Each area of the front would have its own light rail to bring up materiel. The British perfected roll on roll off train ferries to bring fodder and supplies direct from England via train ferries to France. Northern French rail lines were under direct military control of the Army in the area.
By 1917, the Canadians led the way in showing the utility of light railways. Having built thousands of miles of new frontier track in Western Canada in the previous decades, these "colonials", led by J. Stewart, supplied the Canadian Corps who went on to victory at Vimy. From this the light railways were expanded to 700 miles (1,100 km) of track, which supplied 7,000 tons of supplies daily. The ebb and flow of war meant that rail lines were built and rebuilt, moved and used elsewhere, but by the latter years of Passchendaele, Amiens and Argonne, light railways came into their own and pulled for the final victory.