The Metropolitan District Railway (commonly known as the District Railway) was a passenger railway that served London from 1868 to 1933. Established in 1864 to complete the inner circle, an underground railway in London, the first part of the line opened using gas-lit wooden carriages hauled by steam locomotives. The Metropolitan Railway operated all services until the District introduced its own trains in 1871. The railway was soon extended westwards through Earl's Court to Fulham, Richmond, Ealing and Hounslow. After completing the inner circle and reaching Whitechapel in 1884, it was extended to Upminster in Essex in 1902.
To finance electrification at the beginning of the 20th century, American financier Charles Yerkes took it over and made it part of his Underground Electric Railways Company of London (UERL) group. Electric propulsion was introduced in 1905, and by the end of the year electric multiple units operated all of the services. On 1 July 1933, the District Railway and the other UERL railways were merged with the Metropolitan Railway and the capital's tramway and bus operators to form the London Passenger Transport Board.
Today, former District Railway tracks and stations are used by the London Underground's District, Piccadilly and Circle lines.
More information on the Wikipedia page [1] - includes bibliography, here, here, and here.