Prior to placing an order, experiments had been carried out to test various features. A Standard Stock control trailer dating from 1923 had been fitted with a streamlined end. A Tomlinson coupler, which as well as connecting cars together mechanically, also connected the electric and hydraulic systems together automatically, was tried out on two cars, and another had an experimental air-conditioning system fitted. When an order was placed with Metropolitan Cammell for four six-car trains in 1935, variations of all of these features were included in the designs. The trains were supplied as two-car units, permanently coupled together, and three such units made up each six-car train. By careful design, all of the electrical equipment was mounted below the car floor, which meant that the whole length of the motor cars was available for passengers, apart from the drivers cab. A six-car train of the new stock could thus carry as many passengers as a seven-car train of Standard Stock. To meet the second major requirement of improved performance, all of the cars were motor cars.