You may have heard me saying before that a good place to start any new knowledge acquisition is by obtaining a clear definition. By doing so you not only clarify what something is about, you also gain clarity on what it is not. And clarity is critical to avoid confusion and misunderstandings.
Railway Signalling
So for a blog on Signalling Simplicity, a decent starting point is some clarity of what Signalling means. Firstly, let’s be clear about the context here which is railway signalling. This is important as for example a simple Google search for “signalling” delivers first-page results from disciplines as diverse as economics, telecommunications and medicine (“bone remodelling” caught my eye…), and yes also from railways. So, the more accurate term would be ‘railway signalling’, but in order to keep it simple I will keep the shorter form ‘signalling’ and you know now what I mean by that, especially if you are from the railway industry (as most of my readers will likely be).
Next, let’s find a SIMPLE and comprehensive definition of what signalling means. I thought about that a reasonably long time (like, a minute of two) for one of my training courses for non-signalling people, and here is what I came up with:
The definition of Signalling
Signalling are all systems and components that enable collision-free train movements.
This definition ticks several boxes:
- It indicates that signalling is a system comprising a variety of different subsystems and components that interact with each other;
- Signalling is first and foremost about controlling train movements, and enabling those in a safe manner; and
- Safe train operation is about keeping trains apart so that they do not hit each other, but also to keep them safely on the track so that they do not derail and collide with any infrastructure.
Signalling safety is paramount
Train crashes or derailments usually make the news headlines because of their spectacularly severe consequences. Or more colloquially in media speak: “Train wrecks attract eye balls.” This is because of the massive weight of trains and the enormous energy and impact when trains travel at speed and then hit anything. Due to that it is understandable that the safety requirements for systems meant to prevent train crashes (i.e. signalling) are outstandingly high. This explains why conventional signalling systems prioritise safety over performance and often end up being capacity constraints for a railway operation. The wider trains are kept apart, the safer it is, but also the fewer trains you can run on a railway line.
Why trains need signals
It is one of the most important rules in railways that trains must only move when they have been authorised to do so. This is because a train travelling at decent speed will not be able to stop ahead of an obstacle (another train on the same tracks, for example) if it only starts braking once the driver has seen that obstacle. So you need an overview external to the train of where the next obstacle, or danger point, ahead of the train is and when it is safe for the train to move forward to the next control point. That overview function is part of the signalling system, as is the transfer of movement authorities to trains.
Key signalling functions
This indicates some key functions that a signalling system needs to provide:
- Determination of trains locations;
- Identification of obstacles ahead of a train (for example a road crossing where cars traverse the railway tracks);
- Control of the direction a train can take (for example crossover from one track to another);
- Computing all the inputs and outputs (main outputs are signals, we get to that in a moment);
- Planning, commanded and control of train traffic; and
- Transmission of movement authorities to trains and their drivers.
Types of Signals
The classic analogy of signals are traffic lights for car and pedestrian traffic, such as the three red ones shown in the image for this post. The basic indications are the same as in road traffic, red means stop, and green means move. Every other optical aspect is an add-on for better function, for example yellow (or ‘amber’ as sometimes called) aspects for advanced warning of an upcoming red signal. Or a particularity of a local signalling system. In Sydney, for example, signal indications (also called ‘aspects’) are a combination of two different signals mounted on the same mast on top of each other.
Note that ‘traffic lights’ are not the only type of signals, even though they are most popular these days. Any indication to the train driver can act as transmission of a movement authority, or a stop command, including for example:
- A signal with ‘arms’ in different positions (also called ‘semaphore signal’);
- A person waving a flag;
- A person waving their arms in a defined way;
- A ‘token’ given to the train driver (which sometimes can be a simple wooden staff);
- An immobile sign (for example on a buffer stop);
- A verbal authority given over voice radio communication; or
- A display on the driver’s console which is updated by data radio communication with the control centre.
Other elements of a signalling system
It has hopefully become clear in this post that a signalling system is much more than just the signals. For a simple overview, other elements of a signalling system are best structured in different layers.
- The traffic management layer comprises systems used for the planning, commanding and controlling of train movements. Those systems are usually called something like Traffic Management System (TMS), Centralised Traffic Control (CTC), Network Management System (NMS), Automatic Train Supervision (ATS) or similar. To my knowledge, Australia is the only country worldwide where those systems are referred to as ‘Train Control System”. (The potential for confusion from this will be addressed in a future post.)
- The control layer which is responsible for the safe assurance and execution of commands from the traffic management layer and for processing information from trackside components. The key element of this layer is typically called “Interlocking” (which will be explained in more detail in another future post).
- The trackside layer, which comprises all signalling elements mounted on or near the railway tracks. These elements include ‘point machines’ allowing to change direction of train travel at junctions, ‘level crossings’ coordinating road and rail traffic at road-rail intersections, ‘track vacancy detection’ to identify the presence of trains in defined sections of the track, and of course the signals.
- The onboard layer, in cases where parts of the signalling system are onboard the trains, for example the cab signalling display mentioned above.
- And lastly the communication layer, comprising the cable connections between the various elements of a signalling system, and also the track-train communication in cases where the signalling system includes elements onboard trains.
In writing this it became clearer to me how much more could be written for explaining all the above in sufficient detail. For now I hope this is enough of an initial introduction so I will leave more detail for future posts.
