Acton Works is a London Underground maintenance facility in West London, England. It is accessed from the District line and Piccadilly line tracks to the east of Acton Town station, and was opened in 1922. It was responsible for the overhaul of rolling stock, and gradually took on this role for more lines, until the formation of the London Passenger Transport Board in 1933, when all major overhauls of underground vehicles were carried out at the works. By 1985, when rolling stock had become more reliable and maintenance intervals had increased, this function was devolved to depots on each line. Subsequently, Acton continued to overhaul major items after they had been removed from trains at the depots, and tendered for work, which included the conversion of the A60 Stock to One Person Operation. It is likely to be reorganised and expanded to house the departments displaced from Lillie Bridge Depot which is being demolished as part of the redevelopment of Earls Court Exhibition Centre.

The site housed the engineering design department for London Underground, and under the leadership of charismatic figures like William Sebastian Graff-Baker from 1922 until 1952 and Stan Driver in the 1960s, produced a series of elegant and innovative designs for batches of rolling stock. The department's nadir was the design of the 1983 Stock, which was produced without any input from an outside design agency. The stock was less than successful, and resulted in a decision that design agencies would always be involved in future rolling stock development.

Major rolling stock projects carried out at the works have included the refurbishment of 15 of the Metropolitan Railway electric locomotives in 1953, and the conversion of the experimental 1960 Stock for Automatic Train Operation trials on the Woodford to Hainault section of the Central line, in preparation for its use on the Victoria line. The works has also been the home of several works shunters. Two were made from redundant driving motor cars, cut in half and joined back to back. A third consisted of two cars of 1935 Stock, which had been converted at the works in 1969 for articulation trials, and acted as a shunter after the trials were completed. A fourth shunter was built by modifying two driving motor cars of 1938 Stock.

Acton Works was conceived as a central overhaul workshop for the London Underground, and the first part of it was opened in 1922. It was designed to allow the overhaul of around 16 cars per week, and initially serviced cars from the District line, the Piccadilly line and the Bakerloo line. Motor cars entered the works for major overhaul every 50,000 miles (80,000 km), or approximately once a year, while trailer cars were overhauled less frequently, after they had run around 70,000 miles (110,000 km). Subsequently, the works was expanded, to enable it to cope with the overhaul of 60 cars per week. Rather than just the three lines, the cars came from all of the lines run by London Underground, although the Metropolitan line remained independent for longer than the rest. Facilities for the repair of bodywork and repainting were also added. The works did not overhaul cars from the Northern line until 1927, when a new underground junction was constructed at King's Cross to allow transfer of the vehicles, while transferring cars from the Bakerloo line involved a circuitous route via Willesden Junction and Earl's Court, until the line was extended to Stanmore in 1939. Maintenance of Metropolitan line trains moved from Neasden Depot to Acton Works in 1933, when the London Passenger Transport Board was formed.

Cars for overhaul would arrive at a small platform near the trimming shop, where the seats would be removed for refurbishment. At the lifting shop, the car body would be lifted off its original bogies, and mounted onto accommodation bogies, which provided much more clearance below the car, so that items below the car floor could be accessed more easily. A system of haulage chains, mounted in channels underneath the workshop floors, was used to move the car bodies through the various workshops. Dirt and accumulated debris was removed by high-pressure air jets, before insulation testing of the electrical circuits was carried out. A traverser moved the body to one road of the car body shop, where there were facilities for removing and maintaining reciprocating compressors, door engines and valves, brake valves, drawgear, electrical equipment and other items requiring servicing. Cars then moved to the paint shop, until the advent of unpainted aluminium bodywork.

In a separate workstream, the service bogies were stripped down into their component parts. Motors were taken to the motor shop, and when only the frames and wheelsets were left, ultrasonic testing of the axles was carried out. Badly worn tyres were removed, and new tyres fitted to the wheel centres. If the wear was less severe, the wheels were turned on a lathe to restore their profile. Once all the parts had been serviced, the bogies were reassembled on a conveyor, and stored until needed. The motor shop included facilities for complete reconditioning of traction motors, including rewinding armatures, and baking them at high temperatures to drive out all traces of moisture and harden the insulating varnish. Returning to the lifting shop, the motors were remounted in the bogies, and the car bodies were transferred back onto the service bogies, before final testing and the replacement of the seats. The works included a reconstruction shop, which was used for modernising old stock, or altering non-standard cars so that they more nearly matched a particular class of stock. There was also an experimental shop, which produced mock-ups of new designs, and a machine shop, where component parts for the repair or refurbishment of major items were made.

The requirements for major overhauls gradually reduced, as equipment became more reliable. The interval at which cars received a heavy overhaul increased from every four years to every nine. In 1947, the first underfloor wheel lathes were installed, which allowed wheel flanges to be machined without removing the bogies from the cars. In 1961, a lathe was installed at Northfields depot which could reprofile the whole wheel, without uncoupling individual cars, and this became standard practice. Improved insulation used in the manufacture of motors meant that armatures did not need to be rewound, while the use of aluminium body panels meant that the paint shop became redundant. By 1985, much of the equipment at Acton needed replacing, and London Underground took the decision to devolve the overhaul of cars to the depots on each line. The works was still responsible for the overhaul of some equipment, after it had been removed from cars at the depots.

Subsequently, Acton Works quoted for engineering projects, such as the conversion of the A60 Stock to One Person Operation. They won the contract, and most of the work was carried out at the works, although some was performed at Ruislip depot. Although much of the works was disused after 1985, a new Equipment Overhaul Workshop was created in 1989. The following year, a Depot Engineering Support Unit (DESU) opened at Ealing Common Depot, located on sidings at its eastern end, but this was short-lived, and the functions of the Support Unit were relocated back to Acton Works soon afterwards, with the building at Ealing Common being used to store items from the London Transport Museum's collection.