The British Rail Class 378 Capitalstar[5] is an electric multiple unit passenger train specifically designed for the London Overground network.

By the start of the twenty-first century, several types of rolling stock that had originally entered service multiple decades before were in operation across London.

[8] In August 2006, it was announced that a contract worth £223 million had been signed between rolling-stock manufacturer Bombardier and TfL, under which an initial batch of 152 individual cars would be supplied, deliveries of which were to start in September 2008.

[11] On the first public unveiling of the Class 378, TfL announced that it had reached an agreement to procure a further three dual-voltage units, taking the total number to 57.

[13] At the time, TfL's business plan called for five-car services to begin on the East London Line from November 2014, while the rest of the electrified Overground network would follow by the end of 2015.

[16] TfL had originally purchased the fleet outright but, in 2007, it completed a sale and leaseback deal that saw ownership pass to QW Rail Leasing.

[1] The Class 378 Capitalstar is a dual-voltage electric multiple-unit (EMU), designed to be supplied via either the 750 V DC third rail or the 25 kV 50 Hz AC overhead lines.

[8][4] In conjunction with the base internal configuration, the cars were outfitted with numerous modern amenities aimed at improving passenger comfort, such as air conditioning, real-time passenger information systems, wheelchair accessibility (including the facilitating of level access), and different seat moquette colours to highlight priority seating.

[17] The train is equipped with sensors to detect how many people are in each coach, and automatically adjust how much energy is expended on heating a particular carriage based upon this capacity, among other intelligent management functions that are performed.

[8] External CCTV is displayed automatically on releasing the doors via an in-cab monitor, removing the need for Driver-Only Operation (DOO) equipment such as monitors/mirrors at platforms.

[16] In December 2018, unit 378232 was shortened back to four cars to run on the Gospel Oak to Barking line while delays to delivery of Class 710 were resolved.

[24] During January 2018, the Class 378 fleet was temporarily withdrawn from service for urgent safety inspections after one unit suffered a failed brake caliper.