The first train began working on the Piccadilly line on 9 September 1957, and all three were operational by the following April. Trials went well, and the design became the prototype for the 1959 stock. An order for 76 seven-car trains was placed with Metro-Cammell, and the first unit entered service on 14 December 1959. All 76 were in service by 30 March 1962. Visually, they were very similar to the 1956 stock, the obvious difference being that the earlier stock had five lights on the front, which were used to display the destination of the train, and the later stock had two headlights. When the 1956 stock was delivered, it was numbered in a series starting at 40000, but when the 1959 units arrived, they were numbered from 1012 upwards, leaving space for the 1956 stock to be inserted at the start of the sequence once they had been modified to be compatible.