Trench warfare
Trench warfare prevailed on the Western Front from late 1914 until the Germans launched their Spring Offensive on 21 March 1918. After the buildup of forces in 1915, the Western Front became a stalemated struggle between equals, to be decided by attrition. Frontal assaults, and their associated casualties, became inevitable because the continuous trench lines had no open flanks. Casualties of the defenders matched those of the attackers, as vast reserves were expended in costly counter-attacks or exposed to the attacker's massed artillery. There were periods in which rigid trench warfare broke down, such as during the Battle of the Somme, but the lines never moved very far. The war would be won by the side that was able to commit the last reserves to the Western Front.
Trench railways
Trench railways represented military adaptation of early 20th century railway technology to the problem of keeping soldiers supplied during the static trench warfare phase of World War I. The large concentrations of soldiers and artillery at the front lines required delivery of enormous quantities of food, ammunition and fortification construction materials where transport facilities had been destroyed. Reconstruction of conventional roads (at that time rarely surfaced) and railways was too slow, and fixed facilities were attractive targets for enemy artillery. Trench railways linked the front with standard gauge railway facilities beyond the range of enemy artillery. Empty cars often carried litters returning wounded from the front.