The designations “Electrostar” and “Capitalstar” stand for a family of electric multiple units built by Adtranz, later Bombardier, for several British companies. They share some similarities with the Turbostar family of diesel multiple units. In most Electrostar sets, all cars except one have two traction motors in one bogie with 200 to 300 kW. Depending on the class, they are equipped for service under 750 V DC via third rail, 25 kV AC via overhead line or both. More than 600 sets have been built since 1999.

The classes 357 and 379 are built for AC service and have a top speed of 100 mph. The classes 375 and 377 have the same top speed, but can operate under AC and DC. The class 376 is a DC-only variant that only reaches 75 mph. The fastest one in the family is the class 387 that is capable of AC and DC operation and reaches a top speed of 110 mph. The class 378 is the only one that carries its own name “Capitalstar”. It was built for the London Overground and can also operate under AC and DC with a top speed of 75 mph. For better acceleration, it has three traction motors in four of its five cars.