Different track gauges were used in different parts of the world including , , and 1.05 m.
The military light railways in France were of gauge and used a variety of steam and petrol locomotives from French, British and American builders. The Germans installed their 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 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.
A large number of locomotives (mostly of gauge) was ordered for the WDLR. These included:
A few captured German feldbahn locomotives were also used but these usually had short lives because no spare parts were available for them.
Both the French Army and the U.S. Army had their own locomotives, which included:
Probably the most famous of these war service engines were of class 10-12-D, built by the Baldwin Locomotive Works, U.S.A. Nearly 500 were built and those that survived the war found new homes around the world. Many went to India and after the war a few went to railways in Britain including: