corrosion there or in adjacent utility mains. It also means that the two running rails are available exclusively for track circuits. The surface sections use the fourth rail solely for operational consistency: the system shares track with Network Rail in several places. Where the track is shared with 750 V third-rail stock, the central rail is bonded to the running rails and the outside rail electrified at 660 V. This allows both types of train to operate satisfactorily. The suburban network of the London & North Western Railway (LNWR) was electrified in co-operation with the Underground, but during the 1970s British Rail introduced third-rail EMUs and the sections of the LNWR suburban network not used by the Underground had the fourth rail removed (see "London and North Western Railway", above). The Underground has carried out studies to consider raising the voltage above the present 630V nominal. New equipment at their substations does allow for a future increase to a standard 750V nominal. In addition, the electrical equipment of new trains are also based on the use of 750V rated equipment. So, whilst new equipment is being designed to for 750V operation, no decision to increase the voltage has yet been made public by the Underground. 750 V DC, third rail (bottom contact) Docklands Light Railway This uses bottom-contact composite third rail, with an aluminium body and a steel contact surface. The advantage of this is a low-resistance, high-current-capacity rail with a durable steel surface for current collection. The rail may be surrounded by insulating material on the top and sides to reduce the risk of electrocution to railway staff and trespassers. The bottom-contact system is less prone to derangement by snow than top contact. 750 V DC, fourth rail (top contact) Waterloo and City line This system is unique to this line of London Underground operated railways. The use of 750 volts came about because the line was originally owned by Railtrack and operated by Network South East. It was upgraded in 1992/3 for both traction supply and rolling stock. Railtrack upgraded the original three rail system to four rail to solve problems with electrolytic damage to the iron tunnel linings (the reason four rail operation was adopted for all other tube lines). They also changed the voltage to 750 volts which had been adopted as their standard DC operating voltage some years earlier. The line was subsequently sold to