The British Rail Class 323 are electric multiple unit (EMU) passenger trains built by Hunslet Transportation Projects and Holec. All 43 units were built from 1992 through to 1995, although mock-ups and prototypes were built and tested in 1990 and 1991. Entering service in 1994, the 323s were among the last trains to enter service with British Rail before its privatisation in the mid-1990s. The units were designed to operate on inner-suburban commuter lines in and around Birmingham and Manchester with swift acceleration and high reliability. Of the 43 units built, 26 are in operation with West Midlands Trains and 17 with Northern Trains. In the first half of 2024 the West Midlands Trains units are planned to be replaced by new Class 730 EMUs, and 34 units will be operated by Northern Trains. The units are known for the distinctive sounds that can be heard during acceleration or deceleration. These sounds are generated by the traction electronics. Background In 1990 the Regional Railways sector of British Rail tendered an order for new electrical multiple units, both to replace older electric units around Birmingham and Manchester, and to work services on the newly electrified Birmingham Cross-City Line. In June 1990, the contract was awarded to Hunslet Transportation Projects of Birmingham, a new company set up by a team of engineers and managers who had left Metro-Cammell (at that time a Birmingham-based train builder). It won the contract in competition with six other European train builders. The trains were designed in Birmingham, but built and fitted out at the Hunslet works in Leeds. Initially 37 units were ordered, with the option for fourteen more. Eighteen would be needed for the Cross-City Line, while the remainder would replace older units (such as the and ); ultimately a total of 43 three-car units were actually built. When the electrification of the Leeds/BradfordSkipton/Ilkley Airedale/Wharfedale Lines was confirmed in the early 1990s, Regional Railways and West Yorkshire PTE applied to the government for 14 units to add to those already on order. At the time, government spending on the railways was restricted due to the impending privatisation of British Rail and eventually, when funding was not forthcoming, the order was cancelled. Instead 21 second-hand Class 308 units from Network SouthEast were used until new EMUs entered service in 2001. The units are known for a distinctive whine made during acceleration or deceleration, rising/falling through multiple