Signalling Importance and Principle

According to reading material for Institute of Railway Signalling Engineer (IRSE) examination module^[1], it says that there are four principle of signalling. They are:

1. Safe train separation – signalling is to maintain a distance between trains, preventing them from colliding in a head-on (or similar way) collision
2. Control speed of train – signalling is to limit and control the speed of train below a safe level, especially at curves and turnout
3. Route setting – signalling sets a route, i.e. signalling controls all the thing along the track (trackside equipment) so that they will allow trains to travel from A to B
4. Hold the route – signalling to prevent the equipment from changing, or causing the train needing to brake.

I am a bit confused with the word principle, as these doesn’t seem like the main reason why signalling exist. In my view, signalling exists to make money. How? Firstly train can run on time between two points/station, which makes passenger want to use the train. Secondly it prevents accident, where accidents will make passenger NOT wanting to use the train. Passengers equals money. Or more likely, goods transferred through railway, makes money for the railway. So a good signalling system is necessary to make money. However when I thought this way, I understand the meaning of the principle, as signalling’s importance is making the railway run safely and efficiently, but the principles of signalling in delivering those values, are as the four points above.

But they are a bit hard to remember. So here’s my take on simplifying it. We agreed (or at least I came to the conclusion) that signalling ‘wants’ to make it safe, so what is unsafe? Accidents are unsafe and have the adverse effect on the bottom line. What kind of accidents are there? I generalize two possible kinds of accidents; trains can derail, or collide. Derailment generally has less impact than a collision. Derailment AND collision would be the worst. So let’s rearrange the principles based on bad to worst impact, with the main keywords:

1. Control speed – prevent derailment, at curves, at turnout, even straight line too if the speed is too excessive
2. Safe train separation – prevents collision, especially head-on collision as the train behind knows if the train in front brakes, and always have enough distance to brake
3. Route setting – prevents collision, can be also head-on collision, but also side/flank collision, where if a route is not set (meaning the whole track that the train is going to travel is not allocated SOLELY for it), another train might use it or cross the track. Also prevents derailment, as when a turnout is not set, a train travelling through it, will derail
4. Hold the route – prevents collision AND derailment. Similar to point 3, but arguably can be worse. For example if a turnout changes halfway while a train is on it, it can cause the train to derail and split the train. But I am stretching it a bit.

Too bad that point 4 doesn’t have the letter ‘s’ in it, otherwise it would have a cool mnemonic to it. Still, the summary of signalling principles are speed, separate, set and hold.

Hope this short posts make sense and not too grossly incorrect.

Thank you for reading.

1. 2018 Mod1 IRSE Exam_Module 1 Study Guide_v2.0 – IRSE Resources