How did we get here?

Signalling Pioneers

Railway signalling is the system used to control the movement of railway traffic, maintaining safety. It prevents trains colliding or derailing at switches, and operates automatic level crossings to ensure safe separation from road traffic.

Signalling has been around almost as long as the railway. Whilst the very first trains were famously protected by men waving red flags, wayside signalling dates to 1832, just 3 years after Stephenson's Rocket was constructed.

The problem of keeping trains safe is very different to that of, for example, self-driving cars, or air traffic control. Trains move on fixed tracks, meaning they cannot take evasive action. Due to their speed and weight, they have very long braking distances - sometimes measured in miles. 

These unique problems led to a unique solution, achievable with technology available at the time: 'absolute block'. Only one train was allowed to occupy one section of track at a time. Authority to move into a clear section was passed to the driver by a movable ball or flag. Later, a semaphore board with red/green lamp attached (see above) was adopted. 

To prevent a signaller accidentally allowing two trains into the same section, or a train moving across an incorrectly set junction, signals were interlocked with track occupancy sensors and points.

Over time and, sadly, often as a result of large accidents due to a 'hole' in the available safety protection, interlockings developed into very complex electromechanical computers. An army of artisans, technicians and engineers - all experts in their own right - supported design, construction, operation and maintenance. It is a tribute to Victorian ingenuity that many mechanical interlockings are still operational today. 

Interesting fact: Most roller coasters with more than one train running at the same time still use absolute block, with brakes on the track holding carriages back from entering the next track section until it is proved clear.

Coloured Lights

Technology has since evolved, and signalling with it. Through the 20th century, semaphores were gradually replaced by coloured lights, for reasons of simplified maintenance, reduced cost, and increased train capacity. Further accidents led to extra controls to combat hitherto unforeseen edge cases. 

Whilst the very first signalling principles were almost exclusively developed in the UK, as railway technology was independently developed around the world, each jurisdiction began to create its own operating and signalling principles. All had the same ultimate goal of ensuring the utmost safety, but each began to achieve this in different ways. 

The underlying architecture, which proved excellent at ensuring safety and allowing an acceptable level of control, remained unchanged: lineside lights inform drivers whether it is safe to continue at a given speed, or whether they need to brake, based upon some situational processing performed in a lineside interlocking installation. 

Growth in traffic meant that networks and junctions became many times more complex

Simple mechanical interlockings were gradually ruled out in favour of solutions which used daisy-chained electro-mechanical relays. These more complex interlockings necessitated an ever greater specialisation amongst the growing number of experts required for their lifecycle, so much so that they formed their own engineering institution in 1912. Later, in the 1970s-80s, solid state processors (just like in a computer) were deployed into railway control systems in a world first for safety engineering. 

The first colour light systems were revolutionary when deployed in 1898 on London Underground.  Despite the basic idea now being over 125 years old, colour light signals control the majority of signalled track mileage worldwide. Still, as the old adage goes, 'if it ain't broke, don't fix it': and colour light signalling is definitely not broken! So why are Universal Signalling proposing something different? We'll come to that later.


In-Cab Signalling

In the latest signalling systems, the lineside safety computer, the interlocking, is almost exclusively now a specialist purpose-built computer. The computer is programmed with the rules by which trains must drive or take routes. It gives drivers permission to move, and a safe speed. However, rather than sending this information to lineside coloured lights, the information is provided to the driver on a display in the cab via a data radio link. 

There are several types of in-cab signalling available, and whilst other systems exist overseas, the main solutions of note in the UK (where Universal Signalling is based) are RETB, CBTC and ETCS

RETB (Radio Electronic Token Block) was designed as a lower-cost system for long single line sections with low traffic. It controls many parts of the Scottish network like the far north line. Though low-cost, its architecture limits both capacity and operational flexibility, and prevents wider rollout. The latest iteration is RETB NG (Next Generation), which is reportedly much more reliable than earlier implementations.

CBTC (Communications-Based Train Control) was primarily designed as a metro control system where the priorities are short headways between trains to give a high service frequency. Its main drawbacks are its expense, that it requires an integrated cab fitment, and is only really suitable for a captive fleet. 

ETCS (European Train Control System) is the specified UK mainline successor to all existing signalling systems. In the UK ETCS has been re-branded as 'digital signalling', mainly to make an easier soundbite for the media, but this is a misnomer: Signalling has been digital since the very first mechanical interlocking computers in the 1830s. Digital, in this sense, refers to the digital (i.e. transmitting data, not voice) radio link to the train cabs.

ETCS is currently (as of 2023) being rolled out on small parts of the UK network in first-in-class projects. These projects are slated to take decades, and cost what the whole rail industry has acknowledged is a completely unsustainable amount