第三軌条方式
a quarter of the railway track in Great Britain is electrified with overhead line and 14% with third rail. Third rail electrification is increasingly seen as outdated and inefficient, although the rolling stock can be cheaper and the clearances needed reduced. Overhead electrification at 25 kV 50 Hz is cheaper to install and operate, more energy efficient, operates better after snowfall and allows more power to be delivered to the train. An economic case can be made for replacing DC electrification with AC when the trackside equipment reaches the end of its life as long as the rolling stock is capable of conversion and costly infrastructure works are not needed. Some lines, such as the Merseyrail tunnels in Liverpool and the Thames Tunnel in the East London Railway may not be suitable for conversion. As London's suburban trains such the Networker and the Class 455 are not suitable are conversion but the outer-suburban Desiro and Electrostar are, conversion of the outer suburban lines with trains changing mode en route is considered the best option. The third rail electrification between Basingstoke and Weymouth needs renewal in the next ten years. In the 2012 High Level Output Specification the Department of Transport asked the rail industry to present plans to convert the line from Basingstoke to Southampton Docks by 2019 as part of a new electrified route from North to South and as a pilot conversion project.