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Third rail

We have the largest third rail network in the world, powering electric trains, and it’s mostly found on tracks in the South East of England

When we talk about the third rail, we mean the live rail which provides electric power to a train through a conductor placed alongside the rails.

Section of railway track without the third electrified rail
Section of rail track without the third rail

Section of railway showing the third rail, which are darker raised rails in between standard tracks
The darker raised rails – known as the third rail – carry 750 volts of electricity

Nearly half of the UK rail network is now electrified – and more than 30 percent uses a third rail to power the train.

How third rail works

Third rail systems are always supplied from direct current electricity. Trains have metal contact blocks (‘contact shoes’) which make contact with the conductor rail. Conductor rails are mostly steel (a specially conductive type).

Conductor rails have to be interrupted at level crossings, crossovers (a pair of switches that connects two parallel rail tracks) and substation gaps. Tapered rails are provided at the ends of each section, to allow a smooth engagement of the train’s contact shoes. 

Electrical substations

Because the third rail is a very inefficient conductor (most of the electricity we put into it is lost as heat) we have to give the voltage a boost every mile or so and that’s why we have substations.

These take 33kv AC from our railway ring main, convert it to 750V DC and pump it into the rails.

Between these substations are Track Paralleling Huts (TP Huts) which are used for switching between electrical sections.

We had to install many new substations for the Thameslink Programme to allow for more trains to run as the existing substations would have overheated and failed if we had run today’s services on them.

Substation building along the Midland Main Line with machinery next to it
Work taking place to install a new substation along the Midland Main Line

The history of third rail

Third rail systems were installed from the 1920s onwards as in those days it was cheaper and more suited to metro operations.

You vs. Train campaign

The third rail is probably one of the most difficult dangers to see. It looks just like an ordinary rail, but it carries 750 volts – easily enough to kill you. The DC current that flows through is three times as powerful as your home electricity. It’s designed to send power to the train, but you are 70 percent water and the perfect conductor – it will pull you in and not let go until the emergency services are able to switch the power off.

Research shows that in particular a large proportion of young people remain unaware of its potentially fatal dangers.

The You vs. Train campaign, run by Network Rail and the British Transport Police, is targeting teenagers to hammer home the dangers present on the railway, whether obvious like a train, or hidden like the electric current in the third rail or trackside facilities.

The only way to avoid these dangers is to never step foot on the railway in the first place.

Third rail and flooding

Flooding can pose problems on the railway. The lasting damage that flood water can cause to infrastructure can lead to ongoing repair work that takes days, weeks or even months.

When flood water comes into contact with the third rail, it can produce steam – as seen in this video at Mitcham Junction.

An unusual amount of steam can be seen if the weather is particularly cold because the third rail acts as an immersion heater, so there is a temperature differential.

Heavy rain can overwhelm the drainage systems on the railway. When we pump rainwater away from areas with third rail, we always isolate it first.

Our trackside staff work and walk next to the third rail as part of their working environment and they are trained to be safe around it.

Find out more about how flooding affects the railway and what we are doing to reduce it.

Keeping the workforce safe

Network Rail has a clear safety vision: Everyone home safe every day. Conductor rail safety is designed around our staff, who are trained to work on track.

For our staff, in stations and around staff walking routes, the third rail is guarded by kickboards. When our staff are working around the rail, they can use yellow plastic guards that slot over the live rail. In addition, there are hookswitches, which manually remove the cables from the live rail, and Negative Short Circuiting Devices, that achieve the same end using switches in substations.

The live rail is split into electrical sections that can be isolated from each other, while allowing power to flow to other parts of the railway. However, trackside workers are trained to assume the power is always live.

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