Ealing Common Depot is a London Underground railway depot on the District line, located between Acton Town and Ealing Common stations in west London, England. It is the oldest of the main depots on the Underground, having been built in 1905, when the District Railway was upgraded for electric traction. All depot facilities were moved there from Lillie Bridge Depot, and it was known as Mill Hill Park Works. It subsequently became Ealing Common Works, and its status was reduced to that of a depot in 1922, when Acton Works was opened, and took over responsibility for all major overhauls. Most of the functions of Acton Works were devolved back to the depots, including Ealing Common, in 1985.
Extensive remodelling of the tracks around the depot took place in the 1930s, when the route from Acton Town to Northfields was quadrupled, and the local eastbound track from South Ealing station tunnels under some of the sidings in the depot. In 1990, a heavy repair shop was built at the southern end of the depot, but this function was transferred back to Acton Works, and the building is now used for storage of artefacts for the London Transport Museum. Access to the depot by rail can be gained from both tracks at Ealing Common station, where the junction is to the south of the platforms. At the southern end of the depot, a steep incline leads down to the two eastbound platforms at Acton Town station. Road access is from the A4020 Uxbridge Road at the north end, and a service road from the A4000 Gunnersbury Lane leads to the museum depot.
One of the first uses of the depot in 1905 was the storage of redundant steam locomotives, displaced by the new electric multiple units, pending disposal, and a regular steam service between Lillie Bridge and the depot operated for many years, for the transfer of stores. From 1932 a small number of Piccadilly line trains were also stored at the depot, and a small number of District line trains were stored at Northfields Depot, but this practice ended in 1964. The depot is now used for the storage of trains for the District line, and the number of trains required has steadily increased. It also carries out engineering work, and the final commissioning of the D78 Stock took place at the depot from 1979.