 Since 26 September 2012 all services have been provided by eight-car S Stock trains introduced in July 2010 to replace the 1960s A Stock.Part of Bombardier's Movia family, they have air-conditioning, feasible because the sub-surface tunnels, unlike tube tunnels, are able to disperse the exhausted hot air.They have regenerative brakes, returning around 20% of their energy to the network and thus reducing energy consumption.