phases falsely suggestive of a motor connected to a gearbox with a great many ratios, caused by use of a gate turn-off thyristor-based inverter as part of the traction control circuitry that drives the 3-phase AC motors, a common setup in the early to mid 1990s which is notably also present in the Networker family of electric multiple units. The "gear-changing" effect is produced by the simplification of the PWM pulse pattern so as not to overload the thyristor, which switches at lower frequencies than later implementations of the variable-frequency drive and hence produces a lower-pitched sound. Service history British Rail service The Class 323s were initially beset with a number of technical problems related to their traction motors, doors, traction converters, gearbox and vibration at high speed which took several years to resolve, preventing them from entering service. The first unit finally entered revenue-earning service on 7 February 1994. A mixed fleet of elderly diesels which the 323s had been intended to replace as well as some elderly , and electric units were drafted in to operate Cross-City Line services until the problems were resolved. Electric services began on 26 November 1992 on the northern section of the Cross-City Line, before the entire route was energised in June the following year. The 323s became reliable enough to operate a full service in 1995. Post-privatisation service As part of the privatisation of British Rail, all 43 were sold to Porterbrook in 1994 and allocated to the Central Trains and North West Regional Railways shadow franchises. West Midlands Central Trains inherited from British Rail a fleet of 26 units in two blocks; 323201–323222 and 323240–323243. In November 2007, these passed to London Midland when it took over the franchise. In December 2017, West Midlands Trains took over the West Midlands franchise, and the 323s passed to that company. It is expected that they will be replaced by new units on the Cross-City Line in the first half of 2024. To celebrate 30 years service in the West Midlands unit 323221 was repainted into the Centro livery. North West The units were used to replace older stock of Classes and , although some of the latter were retained in reserve until 2000. They are used on the Manchester electrified network, primarily to the south of the city. At the time of the privatisation of British Rail, the Regional Railways North West franchise