 These would be the first vehicles to be designed with air-operated doors.In order to make up complete trains of air-operated stock, twenty "Gate Stock" driving motor cars, built in France in 1906, were reconstructed with air doors, and were normally formed into six-car trains.The cars were the forerunners to the Standard Stock, which became the most prolific group of tube stock to operate on the system, thanks in part to the Trade Facilities Act 1921.