The Great War left an enormous imprint on London and its transportation system. Stations on London's Underground provided much-needed shelter from air raids on London in both world wars. The first ever air raid on London took place on 31 May 1915, prompting the use of Underground stations as shelters. Conditions were basic, but many were willing to cope with discomfort for the relative safety that Tube stations offered.
Londoners also had to get used to the very first use of "blackout"—with streetlights extinguished and internal lighting concealed—to make it more difficult for German airships to find targets. On the surface parts of the Underground network, trains were fitted with blinds or had their interior lights turned off during raids. Half of the male employees of the underground, about 3,000 men, were released to served in the armed forces. This led to the first extensive employment of women as "wartime substitutes" in the system.
London’s searchlights and anti-aircraft defenses initially proved ineffective. It was not until September 1916 that British fighter aircraft were able to intercept and destroy their first airship. From 1917 Germany increasingly used bomber airplanes, particularly large Gotha bombers, instead of airships. Between June 1917 and May 1918 German bombers attacked London 17 times. The Gotha raids of September 1917 put a particular strain on tube stations. There were back-to-back raids on the 24th and the 25th, and another attempt on the 28th, though none reached London in the latter.
Faced with massive influxes of Londoners seeking shelter now almost every night, the Underground issued new restrictions on its stations’ use during air raids. Henceforth, people could only take shelter in stations after the air raid sirens sounded, not preemptively. Additionally, those taking shelter could not bring pets with them. Memories that pets could not be taken into the Underground may have contributed to the massive, spontaneous cull of over 750,000 London pets that occurred after the outbreak of the next war, 22 years later.
In the whole of the war, 667 people were killed and 1,936 injured in raids on London. It could have been many more had the Tube not provided valuable shelter. When the Second World War came, the experience during the earlier conflict made the Underground better prepared for the challenges of the Blitz and the V-weapons.
Sources: London Transportation Museum; Today in WWI: Imperial War Museum