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The rail system is made of multiple technologies and numerous moving parts. Every aspect of the system needs to be carefully monitored to ensure that the trains run smoothly. This is not only important to keep people arriving to work on time but also to keep the commuters and rail workers safe. One of the main safety mechanisms to do so is train signalling.

What is train signalling?

In simple terms, railway signalling is a system of traffic lights for trains. Safety rules permit only one train on a specific section of track at any one time to keep the trains from colliding. Most major railway systems, the UK one included, operate multiple trains per hour, with numerous intersected routes. This is why there has to be a way to control their movement, and that’s where UK rail signals come into play.

The tracks are divided into blocks. So even if certain tracks cross paths, ensuring each block has only one train at a time prevents potential accidents. When one train crosses a block, the other drivers are signalled to wait, slow down or are routed via an alternate section of track, thanks to the signals (traffic lights) on the track's left side.

Railway signalling principles

The signal lights usually have four indicators:

  • Green: clear to go.

  • Double yellow: still clear to go, but indicates that the following signal may be a warning.

  • Yellow: warns that the next signal may be red.

  • Red: means stop or danger ahead. It is forbidden to go when the light is red.

This is necessary to mitigate the high speeds trains can drive at, as unlike road traffic lights the rail signals give an indication that you may have to slow down / stop at the next signal, or that you can continue at full speed, this maintains safety levels but greatly assists with right time train running.

A short history and introduction to railway signalling

Train clashes were not an issue when trains started operating, as only one train was running on each track. But as demand grew and more trains were added, there was a need to develop a signalling system. The first one of its kind was with Railway Policemen, also nicknamed “Bobbies.”

They would hold three types of flags: red, yellow, or green. Those flags indicated how far away the passing train was and if it was safe to pass. Since the policemen could only report on what was in their line of vision, accidents still occurred. There was a need for a better system to have an overview of the entire railway operation.

In 1841, the UK started to use mechanical signals. Instead of holding flags, the Bobbies became lookouts who reported train activities to signal box operators. Those switched the signal with levers from a control centre. At this time, the railway design also started to use blocks to ensure a clear path for each train.

In 1860, the system started to include a yellow warning sign, considered a distance signal. It lets drivers know they must slow down, as the next signal will be red. This allowed for trains to go faster and remain relatively safe.

How does UK railway signalling work?

The next step in the evolution of UK railway signalling was introducing the colour light signals. Most countries use the basic green, yellow, and red, while the UK in particular also has the double yellow indicator. Up until the 1980s, this system was also controlled by human operators. However, when computers started to become more commonplace, more automation was introduced.

Nowadays, much of the rail signals are automated, and run thanks to a system called CBTC (Communications Based Train Control). (MH note I would remove reference to CBTC this Comms Based Train Control and used on London Underground and Metro systems and not heavy rail in the UK)  Drivers are appropriately advises if/ when it’s safe to continue, with the help of automation. There are two main methods that provide the relevant information for drivers.

1) Fixed block

With this method, the system is alerted when a train enters a block of track. The system is alerted when a train enters / leaves a section of track via the train’s bogie completing / removing a circuit between the two adjacent rails, you will doubtless have heard the phrase ‘track circuit’. The alert sends an electric notification to update the signal before the block to red, and the one before it to yellow. Once the train leaves that block, another alert is sent. This time, the signal changes to allow trains to pass. In some instances, this system can even detect there’s an obstruction on the block (via a track circuit completing with no known trains in the area), and sends a red signal to appropriate oncoming traffic.

2) Moving block

Here, a computer calculates a safe distance around each moving train. A combination of sensors and radio signals determine the precise location and speed of the trains. Then, it dictates the safest distance for trains to be from each other. This information is then fed to the trackside signals.

The way that the system works is to always default to a red signal to be as safe as possible. So even if there’s a technical malfunction, they will never show all green. Instead, they will always present red. This instance is commonly known as a signal failure.

Nowadays, with some trains travelling over 100 miles per hour, drivers might not be able to read the trackside signals at all. This is why more and more train operators are looking into improving the system and upgrading to in-cab computer displays.

On top of that, most trains also have on-board warning systems usually one or more of the following (AWS Automatic Warning System, TPWS Train Protection Warning System or ATP Automatic train Protection. These systems assist the driver to ensure that the train runs at the correct speed and obeys the signals (traffic lights). The systems can automatically trigger the train’s brakes if required).

Why is rail signalling important?

As already stated, rail signalling is the primary way to avoid train collisions. It does so by ensuring a safe distance between trains and only allowing one train on a track block at a time.

There are many variables to consider that potentially increase the risk train safety issues and can cause delay to your journey.

  • A person or an object falling on the track.

  • On track trespass / vandalism

  • Theft of rail cables / equipment

  • A train that breaks down in the middle of the route.

  • A medical emergency on one of the trains, which requires it to stop.

  • Partial route closures due to asset failure / engineering work.

  • Congestion on the line causing delays.

It’s important to remember that train drivers travel on a fixed section of track cannot swerve out of the way like regular car drivers. The only way they can normally operate is forwards    (in certain instances a driver will be given instructions to change ends and drive in controlled conditions back the way they came for a certain predefined distance). So any small obstruction can result in a significant delays and potential safety issues. In addition, the faster a train goes, the more space it needs to break.

And for the railway system to be efficient and punctual, it can’t afford to have too many stops and adjustments. Rail signalling is a proven safety system that keeps passengers safe while ensuring they arrive at their destination as close to on time as safely possible.