The British Rail Class 309 "Clacton Express" electric multiple units (EMUs) were built by British Rail (BR) York Carriage Works from 1962–1963. They were initially classified as Class AM9 before the introduction of TOPS. These units were the first express 25 kV alternating current (AC) units to be built by British Rail and were their first EMUs capable of 100 mph. Description and design Twenty-four units were built in three different configurations: 601–608 – Two-car units (309/1) 611–618 – Four-car units containing a griddle car (309/2) 621–627 – Four-car units (309/3) Each unit had identical electrical equipment. The original concept called for increasing the power-to-weight ratio when strengthening trains from eight to ten cars in peak periods using the 2-car units, in order to make the peak timetable more resilient. Due to problems that had been encountered with the BR1 bogie design then current for Mk1 loco-hauled coaches, the Class 309s were built with Commonwealth bogies, much heavier than the more modern design, but more robust. The exception to this is vehicle E19608 which was originally a BR Class 123 Buffet Vehicle No. W59831. This was converted to a 309 griddle car but retained its B4 bogies, becoming the only vehicle in the class with this type. This vehicle also had 36 seats in the Buffet area compared to 24 in the other units. More unusual was the front end design, incorporating the drivers cab but retaining passenger access between units via a pullman gangway, particularly necessary to allow access to the griddle car from all coaches. The design was based on the front end of the Class 303 but with the gangway placed centrally and also incorporating a headcode box. Also notable was the curved wrap-around glass on the cab windows, but these proved expensive to replace and later an additional pillar was inserted so that flat glass could be used. The units were originally class AM9 (AC Multiple Unit) pre-TOPS, and were painted in the BR standard coaching stock maroon livery lined black & yellow, with the driving end gangway doors painted warning panel yellow. Units 605-608 were expanded to 4 coaches in 1973 by the addition of a Corridor Second (SK) and a Corridor Composite (CK) and reclassified 309/4. In 1981 units 601-608 were again altered so that the formation became MBS-TC-DMS, and numbers 605-608 were reclassified back to 309/1. Operations When built, units were originally planned