Under North London, there exists a private underground monorail service, some 20km long, running from Elstree to St John’s Wood in the centre of town. You can’t ride on the monorail, and it’s not an escape route for Royals or rich oligarchs, it’s actually owned by the electricity grid — and it’s their inspection railway.


Fata Automation monorail

Completed in November 2005, the deep level tunnel, some 25 metres below ground connects a new 400 kV substation at St. John’s Wood to its counterpart in Elstree and was needed to meet London’s ever-growing demand for power.

This 20km long tunnel contains one of the longest 400kV XLPE cable circuits to be installed in Europe. This tunnel presently only contains one 400 kV cable circuit but has the facility to have a second installed in the future.

Construction of the 3-metre wide tunnel took place from April 2001 to March 2004. To put the numbers into context, a typical tube tunnel is only slightly wider, at 3.5 metres.

The tunnel was constructed utilising three tunnel boring machines (TBMs) working from two drive shafts. From the Elstree drive shaft, the tunnel was driven south to Canons Corner (5.6km). Initially, the strata were sand and gravel; however, this gradually changed to London Clay as the tunnel deepens towards shaft 2.

At Cricklewood, the tunnel was driven in both directions, northwards to Canons Corner (8.2km) and southwards to St Johns Wood (6km). The tunnel in this section was constructed wholly in London Clay using an expanded lining. Access shafts were either 7.5m or 10.5m in diameter and up to 42m deep – all sunk by underpinning methods.


The end of the tunnel

But, when you have a 20km long tunnel, how do you inspect it?

Walking is out of the question, for duration, and in close proximity to the electricity cables, human safety reasons — so an unmanned battery-powered monorail was installed.

Supplied by Fata Automation, the monorail network supports 4 remote controlled cable inspection vehicles.

The vehicles are designed to remotely inspect the National Grid cables within the tunnel using infrared cameras fitted to the vehicles. The vehicles are controlled remotely at control desks fitted with television monitors which allow the operators to control the vehicles and view the condition of the cables.


The monorail tracks run along the top of the tunnel

A major tunneling project has been adding to London’s electricity capacity over the past few years. And all of those tunnels will contain small railway lines of some type for inspection trips.

While we think of the tube and railway tunnels as the major part of underground London, there is an entire “shadow” tunnel network of sometimes even larger proportions under the streets – telecoms, gas, water, sewage and of course, electricity.

Not just tunnels, but also substations, which are sometimes concealed under new office developments, such as this one in Victoria.



National Grid press release

National Grid case study

University of Southampton presentation

Murphy Group

Fata Automation


Energy for London


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  1. Marie-Eve says:

    Interesting! Next to the cable inspection tunnels, the disused tube, the royal mail rail which others are there?

    • Jeremy Löbl says:

      The ‘fast Northern line’ is another. Then the Kennington loop. Another loop just south of Embankment.

  2. Francesca Fenn says:

    But I really, really want to ride on it. Please fix it for me, Ian!

  3. Dave H says:

    In which case what contribution from National Grid to building rail & utilities tunnel along South Bank – Battersea to Bermondsey

    As with 400KV beside Regents Canal this could use the Thames for cooling circuits.

  4. Arthur Wankshaft says:

    The article gives the impression that the cars are un-manned. This is not entirely correct as each car/trailer pair can carry up to 5 personnel along with 500kg of equipment. However, operation of the vehicles can be undertaken remotely if necessary. Top speed is about 5.5mph though so don’t expect a white knuckle ride!

    • woodward says:

      The cars are unmanned initially. If a problem is detected then a repair team can use the car to transport them to the area.

  5. Michael Rowell says:

    Seeing as most utilities are buried UG, why do they need a monorail to inspect these cables? There must be more to this than meets the eye. I used to live very near Elstree which is on a hill and there were plans for the Underground Northern Line to go there post war. I can remember the blowing up of the installed bridges near Spur Road roundabout when I was young!

    • Ian Visits says:

      High voltage electricity cables generate heat which can cause problems if there are imperfections in the cable — it’s normal practice to keep a close eye on the high voltage cables as a fault can start a fire — not ideal even above ground, but could be catastrophic in a tunnel.

      Not to mention the loss of power to a large chunk of London.

  6. Ronnie Williams says:

    You might find this interesting…its not just down in the SE that the HV network is being strenghtened! National Grid and ScottishPower Transmission have come together in a joint venture to build the Western Link, a £1 billion project which will help to bring renewable energy from Scotland to homes and businesses in Wales and England.The Western Link project will transmit 2,200 MW of power, sufficient electricity to meet the needs of around 2m people, or a city four times the size of Liverpool.http://www.westernhvdclink.co.uk/the-project.aspx

  7. raynette says:

    How do people get down there? Where is the entrance to the tunnel?

    • Ian Visits says:

      To get down there you get a job working for the National Grid and start working on looking after the tunnels.

  8. misar says:

    Michael Rowell says:
    October 15, 2016
    There must be more to this than meets the eye. I used to live very near Elstree which is on a hill and there were plans for the Underground Northern Line to go there post war. I can remember the blowing up of the installed bridges near Spur Road roundabout when I was young!
    The abandoned “Northern Heights” extension of the Northern Line to Elstree and Bushey Heath was a surface line. It did tunnel under the hills near Elstree but this was like the tunnel between Hendon Central and Colindale, not a deep level tube. Anyway, the route was different from the electricity tunnel.

    • Michael Rowell says:

      misar,thank you so much for that info. I grew up in Edgware as a lad and wasn’t aware of all of that.

  9. Keith Beattie says:

    Wondering if the tunnels themselves are cooled down at all, there must be a huge amount of heat generated by the cables alone.

  10. Nigel Spate says:

    Is there a fire extinguisher systems in the tunnel?

  11. Ian prince says:

    Used to work for BT they have deep tunnels in cities to get cables to the exchanges, it was said they were to prevent bomb damage. I only went down one under marks and spencer in Peterborough .

  12. Clive says:

    Do we know where the Elstree end is?

    • Patrick says:

      As far as I am aware the tunnels run:

      West to East: Willesdon (*approx 8 miles south of Elstree*), Kensal Green, St John’s Wood, Hackney. [19.9km]

      It also runs North to South (not mentioned in this article) from Kensal Green to Wimbledon [21.1km].

      …. in all there are 32km of tunnels ranging in depths from 12m to 60m. With more proposed.

      EnergyforLondon (as referenced in sources) has a project profile:

  13. saxbophone says:

    A bit ironic that they use battery power despite being at the site of a main power grid route! Of course, tapping power from those cables within the tunnel is out of the question, but still it’s funny!

  14. Ian green says:

    Well if nuclear war breaks out
    I know where I’m going

    Also used to live in Stanmore and I definitely heard the rumble of the monorail ..

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