a day being cancelled due to its aging 1938 Stock and poor industrial relations at that time at Acton Works. The Northern line urgently needed newer trains and 30 more than would be available from the Piccadilly line. The first batch of 1972 Stock, known afterwards as Mk I stock, was ordered to fill this shortfall. Rapidly designed, this was the 1967 stock design adapted for operation with a driver and guard, entering service on the Northern line between 1972 and 1973. The Jubilee line needed 33 trains and a second order of 1972 Stock, known as Mk II, was ordered. Entering service on the Northern line between 1973 and 1974, these differed from the Mk Is as it was planned to convert the line to ATO. These were transferred to the Bakerloo line in 1977. When the Jubilee line opened in 1979 it used the 1972 Mk II stock while the Bakerloo line had 36 trains of refurbished 1938 stock. The 1973 Stock ordered for the Piccadilly line was a new design. Instead of seven cars, the trains were composed of six cars, each about longer than the 1959 Stock cars. This allowed a symmetrical train with fewer cars and bogies. Space for luggage for passengers connecting with flights at Heathrow was left beside the doors, leaving 44 seats in each car. Estimating that the remaining 1938 Stock needed replacement by 1984, in 1979 London Transport proposed building 33 trains for the Jubilee line, releasing its trains for use elsewhere on the system. After some negotiation 15 trains were ordered with an option for a further 13. The resulting 1983 Stock was a development of the 1973 Stock, with six of the longer cars, but with single leaf doors, similar to the D Stock that had been introduced on the District line. Trains entered service between 1983 and 1985, with each new delivery triggering a cascade with a 1972 Mk II moving to the Northern line, replacing a 1959 Stock that moved to the Bakerloo line, allowing a 1938 Stock train to be scrapped. In 1986 passenger numbers were increasing and a further trains of 1983 Stock were ordered. These entered service in 1987–88. After agreement about One Person Operation (OPO) was reached with the trade unions in 1983, work began converting trains and lines. The 1972 Mk II, 1973 and 1983 Stocks were able to be converted and