The British Rail Class 319 dual-voltage electric multiple units (EMU) were built by BREL York in two batches in 1987–88 and 1990. The trains were introduced for new north-south cross-London services from Bedford to Brighton, and since privatisation these services have been operated by Thameslink and First Capital Connect, the former franchise having been merged with the Great Northern section of the former WAGN franchise to form the latter train operating company on 1 April 2006 as a result of re-franchising. Class 319 units have dual-power pick-up, from either 25 kV alternating current (AC) overhead lines for services north of London, or 750 V direct current (DC) third rail to the south. However, some units were used only on outer suburban services in South London. The Class 325 postal units were based on the Class 319 units, with the same traction equipment and body design, but are fitted with cabs of the same design as the newer Class 365 and Class 465 Networker units.

In the 1980s, there were plans for a rail service that would link Bedford and Brighton. These services would cross London in a north-south direction, and thus became the first route for many years to cross London from north to south. These services were branded Thameslink by Network SouthEast, which operated the services. Before the Thameslink service became operational in 1988, stations along the Midland Main Line north of London were served by Class 317 electric multiple units, introduced in 1981. This required the electrification (using 25 kV AC overhead wires) of the line between Bedford and London St. Pancras and the branch to Moorgate. This service was therefore known colloquially as the "Bed-Pan" service. Key destinations included Bedford, Luton, St Albans, Moorgate, and London St. Pancras. As the Thameslink service was to use a route with 25 kV AC OHLE north of Farringdon and along the branch to Moorgate, and 750 V DC third-rail electrification south of Farringdon, the Class 319 trains were built with dual-voltage capabilities, making them very versatile.[1] Their body shape is slightly different from contemporary electric units due to restrictions in the loading gauge in Kings Cross tunnel, which meant that other dual-voltage units were not suitable.[citation needed] They were also required to have Emergency end doors in the cabs,[2] due to the twin single bore layout of Smithfield tunnel preventing normal train evacuation. Two sub-classes of Class 319 units were originally built. Over the years, units have been refurbished, creating five sub-classes, of which four still exist.