Railroads and railways were probably the most developed technological wonder of WW I. Before the war, the German military invested heavily in these trains, known as iron horses, and the tracks that carried them.
The German military had counted on trains and practiced mobilization of its army on them, and they perfected it close enough so nearly no problems existed. The military control of the railroad allowed the German Army to be completely mobilized and on the French/German and the Belgium border in a matter of only two weeks.
The French military did not control its nation's railroads and little practice was done with trains, so the French Military mobilization took over a month to coordinate. This was the strategic advantage Germany needed to allow the Schlieffen Plan to work, because it caught the French and Belgium forces completely unready.
Besides the standard gauge track, which most commercial trains ran on, there was the 60cm, or narrow gauge track. Before WW I, this track was used mostly by farmers to move their harvest to a market or standard gauge railway.
This track was used heavily by both the Allies and the German during the war. The railroad lines were used to supply the front line military units with food, ammunition, and some comfort items. The 60cm gauge railway used two basic types of trains. There was the coal burning locomotive, which was little more than a smaller version of a regular train.
The main problem with these 'little' locomotives was that they had a tendency to tip-over on curved rails. "Probably the worst was their high center of gravity. They were so unstable, that if the locomotive started to lean to one side on rough track, and the water in the side tanks rushed to the low side through the equalizing pipe, the locomotive often tipped over."(Dunn, Richard. Narrow Gauge to No Man's Land, 1990).
The other was a gas mechanical locomotive, called a tractor. The tractor was only able to pull one car at a time, but was heavily used near the front lines to avoid the smoke puffs of the coal fueled locomotives. Narrow gauge was used because it was easy to build, to hide, and repair.
The American Expeditionary Force (A.E.F.) sent five engineer units to Europe with U.S. combat forces maintained trains and worked to build railways to assist the Allies. The narrow gauge may have been small, but in one week it was able to haul 457,833 tons per kilometer.
While trains and railway were not the most glamour weapons on the battlefield, they made it possible to conduct war on such horrible grand scale. Without the iron horse there would have no way to efficiently move, feed, or re-supply all the Armies in the field.