other trains fail. These operate as four, eight or 12-car sets. An additional three units, all Class 321/9s, were transferred to Greater Anglia from Northern Trains in 2020 in order to provide capacity during the introduction of GA's new Class 720 fleet. Former operations Silverlink / London Midland / London Overground Silverlink inherited 37 of the 48 Class 321/4 units. In September 2003, Silverlink's class 321 units were temporarily withdrawn following the discovery of loose bolts on brake discs in some units. In September 2004, London Euston to Birmingham local services were divided into two separate services: Silverlink retained London to Northampton services, while fellow National Express subsidiary Central Trains took over all local services between Northampton and Birmingham. Initially, Central Trains hired Class 321 units from Silverlink to work their new services, but the arrival of Class 350/1s meant that very few Class 321 units were then required. To accommodate this sub-lease, three Class 321/3 units were transferred from National Express East Anglia to Silverlink. These were not permitted north of Rugby due to lack of the National Radio Network system (Eastern Region units only had the Cab Secure Radio System used for Driver Only Operated Passenger services) and therefore could not be used on Central Trains services (although they occasionally appeared on services to Rugby at weekends due to the line being closed between Rugby and Birmingham New Street). This ceased in late 2005 with the introduction of Class 350 trains on the Birmingham to Liverpool route. One of these sub-leased units was involved in a low-speed derailment at Watford Yard. In May 2007, Central Trains began using Class 321 units on some morning services from Birmingham New Street to Walsall. In November 2007, London Midland took over operation of the Class 321 fleet previously used by Silverlink and Central Trains. An order was placed by London Midland for 37 Class 350/2s to replace the Class 321s. London Midland withdrew most, retaining seven units (411–417). They were used for peak hour workings between London Euston and Northampton and on the Abbey line. In 2015, all seven were withdrawn for transfer to Abellio ScotRail to operate on Glasgow suburban lines, with the last withdrawn on 20 September 2015. They were replaced by Class 319s. Two (413/414) briefly operated for London Overground Rail Operations in late 2015, being revinyled in London Overground livery. The cascaded Class 321s were reformed to