 Seven car trains using the 1903 motor cars were considered under-powered, but an only few peak-hour seven car services were operated with Ealing Stock.Stock with air-doors first entered service on the London Underground in 1921, and from 1923 Standard Stock operated on the Hampstead line, and these allowed staff to be reduced to front and rear guards and a driver.A Central London car was experimentally rebuilt with air-doors in 1925, and by 1928 all cars had been converted, the work being carried out at the Union Construction Company at Feltham.