Class 4000 is the name given to the fleet of 20 new diesel multiple units (DMUs) which are to be procured by Translink (Translink (Northern Ireland)). None of the rail network in Northern Ireland is electrified, and the use of DMUs goes back to nationalisation. The units will be a development of the Class 3000 (NIR Class 3000 (C3k)) units, and will be constructed by CAF (Construcciones y Auxiliar de Ferrocarriles). The fleet will be made up in 3 car formations, although the contract between Translink and CAF allows for the purchase of an additional 20 vehicles that will allow the trains to be extended to 4-car units. In addition to this, it is planned that they could potentially be used on the Belfast-Dublin main line (Dublin-Belfast railway line) as alternative stock for Enterprise (Enterprise (train)). Included in the specification is a requirement that the new trains be compatible with the existing Class 3000, allowing them potentially to work in multiple. Construction of the first unit had begun at the start of June 2010.