War I in 1914 delayed construction, trains reaching Queen's Park in 1915 and in 1917. An extension of the Central line west to Ealing was started in 1913 and, also delayed by the war, was completed in 1920. The war saw growth in traffic and a shortage of men, so women were recruited as temporary replacements in traditional men's jobs such as guards, clerks, painters and cleaners. London saw its first air raids in 1915, and people used the tube stations as bomb shelters. After the war new trains were purchased to run on the Metropolitan, District, Bakerloo and Piccadilly lines, the Piccadilly line trains having air-operated sliding doors. In the 1920s, taking advantage of government backed financial guarantees for capital projects that promoted employment, there were major extensions of the City & South London and the Hampstead lines. The tunnels of the City & South London Railway were rebuilt to have the same diameter of the other tubes, and then extended north from Euston to a junction with the Hampstead line at Camden Town, and south to Morden. The Hampstead line was extended to the north from Golders Green to Edgware and south to another junction with the City & South London at Kennington, this opening in 1926. Although physically connected, the lines were still officially named the City Railway and Hampstead & Highgate line. The lines were to be known as the Edgware, Highgate and Morden and the Morden-Edgware until finally renamed the Northern line in 1937. Also during the 1920s the original tube gate stock was replaced with 1,460 cars of Standard Stock with air-operating sliding doors, except for the Central line where the cars were refurbished. Busy central London stations were modernised with escalators replacing lifts. Unlike other railway companies in the London area, the Met was able to develop land for housing. After World War I they promoted housing estates near the railway with the "Metro-land" brand and nine housing estates were built near stations on the line. Taking advantage of the Treasury guarantees electrification was extended north from Harrow to Rickmansworth and a short branch opened from Rickmansworth to Watford in 1925. In the 1920s more powerful electric and steam locomotives were purchased and Metropolitan Railway stations were redesigned by their architect Charles W Clark. With finance guaranteed by the government the Piccadilly lines and Metropolitan were extended in the early 1930s. The Metropolitan