From the website of London Transport Museum:
“When it opened in 1907, Down Street Underground station was like many built across the network in that period. With its distinctive ox-blood red tiles, semi-circular windows and tiled ticket hall, it was typical of the style of Leslie Green, the Underground’s in-house architect in the 1900s. But in contrast to most central London Underground stations, Down Street was never busy. It was in close proximity to Dover Street (now Green Park) and Hyde Park Corner stations, with its depth making it relatively inconvenient to use. The affluence of its local residents, many preferring private transport, also contributed to it being under-used. This led to its closure in 1932.
The very reasons for Down Street’s closure as a station made it ideal for a different purpose in the Second World War. In 1938, the Railway Executive Committee (REC) was established to run Britain’s mainline railways in the event of war. With this task vital to the war effort, a central headquarters was needed safe from the anticipated use of air raids. As Down Street was disused, 22 metres underground and in a central location, it was perfect for the REC’s purposes. Conversion began in 1939.
The narrow tunnels of Down Street station were converted into a variety of rooms. At the heart of the REC’s operation were meeting rooms, offices and a telephone exchange that connected the subterranean headquarters to key government departments and railway locations. There was also accommodation for the 40 staff, including dormitories, washrooms, toilets and dining facilities. The REC operated around the clock and to avoid attracting attention, most staff lived, ate and slept in shifts. Senior executives benefitted from an executive mess room and private bedrooms, both of which were to be put to use by a very significant visitor. Down Street continued to act as the REC’s headquarters until 1947, managing the country’s railways and the movement of troops and equipment.
During a critical phase of the near-continuous air raids of the Blitz in 1940-41, Prime Minister Winston Churchill temporarily used the REC headquarters at Down Street as a safe haven. In a period when 10 Downing Street and the underground Cabinet War Rooms were undergoing repairs and reinforcement, Churchill utilised what his Assistant Private Secretary, John Colville, described as ‘the safest place’. Down Street was also appealing because the food and drink was supplied by railway hotels. In November 1940 Churchill dined with members of the REC and War Cabinet in the executive mess room. Despite rationing, they were served cigars, caviar, vintage champagne and brandy.”