The L Stock trains were built to provide additional rolling stock for the eastward extension of the District Line from Barking to Upminster.[1] Two new electrified tracks were added parallel to the existing steam-operated LMS lines, including several new stations such as Upminster Bridge.
Forty-five cars were built by the Union Construction Company, eight were driving motor cars and the rest were trailers. An innovation for surface stock was the provision of a sliding door at the guard's position.[1] The eight motor cars, numbered 700-714 (even numbers only), were initially owned by the London Midland and Scottish Railway.[2]
The conversion from L Stock to Q31 Stock took place in the late 1930s, with the major modifications being the conversion of the hand-operated sliding doors to air operation and the provision of electro-pneumatic brakes.[3] The last Q Stock trains were withdrawn in 1971.