discontinued across the Underground network at the introduction of the A60 and A62 Stocks, their capabilities of carrying shorter units continued the operation. In June 1962, four-car trains began to run on the Metropolitan line. Decouplings were normally scheduled at Uxbridge or Watford in the platforms, and at Amersham in the sidings. The practice was discontinued after the 1980 summer season; with exception for the Chalfont & Latimer–Chesham shuttle, which was always operated by a single four-car unit; as it had become an operational nuisance. Uncoupling was usually cancelled during each winter as it was hazardous in leaf-fall and icy conditions. This also greatly simplified subsequent rolling stock modifications. When built it had a top speed of (world's fastest fourth-rail train), but from the late 1990s/early 2000s it was restricted to to improve reliability. Other areas of operation In addition to the Metropolitan line, A Stock was permitted to traverse the following sections, subject to the following restrictions: Roster Cars had a four-digit number. The first digit identified the type of car (driver or trailer), the last three digits the set (000 to 231). In further detail: ^ operational DM † Renumbered + Replaced/formed with a renumbered car Renumberings: a. 5034 and 5008 swapped numbers in July 1985; new 5008 and 6008 became set 5234-6234 in September 1994, 5034 preserved at the London Transport Museum, Acton. b. set 5009-6009 renumbered 5235-6235 in September 1994. c. set 5028-6028 renumbered 5232-6232 in June 1985. d. 5036 renumbered 5116 in April 1993 (original 5116 scrapped in 1987 – collision at Kilburn, December 1984) e. set 5117-6117 renumbered 5233-6233 in August 1985; set 5037-6037 renumbered 5117-6117 in April 1993. f. 5208 renumbered 5218 in August 1992 (original 5218 scrapped in 1994 – experimental suspension) g. 5209 renumbered 5121 in March 1993 (original 5121 scrapped in 1994 – collision at Neasden depot, October 1986) Converted works vehicles: s. Sandite dispenser car. Withdrawals: 1. 2. Withdrawal A60 Stock reached 50 years of service on 12 June 2011. The age of the stock made spare parts more difficult to obtain and vehicles had to be cannibalised to keep the rest of the stock in operation. S8 Stock replaced the stock, with the first unit introduced on 31 July 2010. This is similar to the S7 Stock for the District, Circle and Hammersmith & City lines; the main differences are the layout and number of seats and the