These units are “dual voltage” in that they can operate on 25kV ac overhead and 750v dc 3rd rail systems. They were built in 1987 – 1988 for the Thameslink line, operating on the overhead line north of London, and changing to 3rd. Rail at Farringdon to operate in the south. Their 100 mph capability makes these very versatile units, hence the range of seating capacities ranging from “inter city” to suburban. Seven units were turned into Southern “Express” units for fast services to Brighton and feature a small area of “bar” type seating in the pantograph area, the HT cable being enclosed in a “feature” column. The majority of units work the Thameslink route. Which itself has inner suburban and outer suburban configurations. The units are part of the steel bodyshell series based on the Mk3 coach, but feature only a front end escape door (for single bore tunnels) rather than a full gangway. Electrical equipment is GTO chopper type feeding four GEC G315BZ motors, providing 268kW each. The units are currently being refurbished.