FOI request detail

Freedom of Information request - About the technical information and data on electrification system of London Underground

Request ID: FOI-1862-2122
Date published: 03 December 2021

You asked

Dear Transport for London, I wish to get the full explanation on the rail electrification system for London underground and the mode of electrification for different lines. Also, it will be great if you have data and images for the electrification system.

We answered

TfL Ref: 1862-2122

Thank you for your request received by us on 10 November 2021 asking for information about the rail electrification system for London underground, plus associated images.

Your request has been considered in accordance with the requirements of the Freedom of Information Act and our information access policy.  I can confirm that we hold the information you require. You asked for:

the full explanation on the rail electrification system for London underground and the mode of electrification for different lines.

We can advise that the lines are electrified with a four-rail Direct Current (DC) system. The configuration and potential of the conductor rails varies across the network. As of 2020, there are three different conductor rail configurations:

The original configuration is nominally 630 V (voltage), with a −210 V centre conductor rail and a +420 V outside conductor rail. This is the default configuration wherever deep-level tube trains operate.

Beginning in 2016, sections of the sub-surface network were reconfigured to a 750 V configuration (−250 V and +500 V rails). As of 2019, the entire sub-surface network uses this configuration except for Uxbridge to Finchley Road (via Harrow) as 1973 Stock and 1996 Stock (of the Piccadilly and Jubilee lines respectively), share the railway.

In areas where London Underground and third rail rolling stock share tracks, the centre conductor rail is electrically connected to the running rails. This still results in a 750 V voltage but in a 0 V/+750 V configuration. Lines configured in this manner include:
  • Between Gunnersbury and Richmond on the District line, shared with the London Overground between East Putney and Wimbledon on a section of the District line that was transferred from British Rail
  • Between Queen's Park and Harrow & Wealdstone on the Watford DC line, served by the Bakerloo line and London Overground trains.

The four rail system was first used in the early twentieth century. The isolated traction current return allowed a train's position to be detected using DC track circuits, and reduced any earth leakage currents that could affect service pipes, telephone cables, or cast iron tunnel liners.

The traction current has no direct earth point but there are two resistors connected in series across the traction supply. The positive resistor is twice as great as the negative resistor, since the positive rail carries twice the voltage of the negative rail. The junction point of the resistors is earthed, establishing the reference point between the positive and negative rails by voltage division. The resistors are great enough to prevent large currents flowing through the earthed infrastructure.

Also, it will be great if you have data and images for the electrification system

Please see the following link which provides a semi-geographical overview of the London Underground network with the detailed track layout in place at the time these images were produced at:

In accordance with section 21 of the FOI Act, we are not obliged to supply you with a copy of the requested information as it is already accessible to you elsewhere.

If this is not the information you are looking for, or if you are unable to access it for any reason, please do not hesitate to contact me.

Please see the attached information sheet for details of your right to appeal as well as information on copyright and what to do if you would like to re-use any of the information we have disclosed.

Yours sincerely

Jasmine Howard
FOI Case Officer
FOI Case Management Team
General Counsel
Transport for London

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