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.

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.