The A03 stock was a prototype electric unit which was constructed for the Metropolitan District Railway (MDR) by Brush Traction in 1903 for testing on the Metropolitan railway, and Metropolitan District Railway (MDR).

The stock consisted of 14 carriages, 4 End Driving Motors located at the end of each unit, 2 Middle Driving Motors located in the middle of the unit, and 8 trailers, with two trailers later being converted into Control Trailers.

Initially operating between South Harrow and Acton Town, the first train entered service on 23 June 1903. From 1905 the A03 stock served the South Acton to Hounslow service, with this being extended to Uxbridge in 1910. The last carriage to be withdrawn was in 1925, there where no examples preserved.

The stock was constructed of a wooden body and led to the development of the B05 stock. However, the A03 stock was not compatible with later builds.