According to Transport for London (TfL), the city's Underground carries 1.2 billion passengers a year over 402 km (250 mi) of track, with some stations handling 89 million passengers annually. That adds up to a fleet of trains covering 76.2 million km (47 million mi) and an energy bill that defines "enormous." In an effort to make the system greener and cheaper, the authority carried out a five-week trial of a regenerative braking system billed as a "world first" that could slice 5 percent off London Underground's energy bill and save up to £6 million (US$9 million) per year.

The new technology uses an "inverter" energy capture system that recovers energy from the train's brakes and feeds it back into the mains in the form of electricity. In hybrid and electric cars, such recovery systems can help improve range, but in large heavily-packed passenger trains running regularly in one of the most heavily traveled cities in the world, the effects can be significant.

The trial was carried out at the Cloudesley Road substation on the Victoria line and was part of a system-wide modernization program that involves repairing the existing infrastructure, and introducing updated technology, new rolling stock, and new lines. In the test, in one week, the system recovered enough energy to power Holborn station for over two days per week. According to TfL, the technology can recapture up to 1 MWh of energy per day, which could run 104 homes per year.

TfL claims that the new technology could reduce the Underground's carbon footprint, but another bonus is the knock-on effect of recovering energy. Normally, braking a train turns the energy into heat, which gets trapped in the Underground's tunnels and needs to be carried away by air conditioning systems. The inverter technology means less heat, less need for air conditioning, and greater energy savings.