6. Intro to track circuits

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What are track circuits?

Track circuits are the primary signalling system used in Australia – that is, they detect when trains enter, occupy, and leave a ‘block’ or section of the track, and send this information back to the Signal Operator in the Network Control Centre. The train's passage over the track circuits triggers a lights system a lot like your everyday red, yellow, and green traffic light.

Signalling and electrical engineers make careful calculations taking into account the length and speed of the train, as well as what is referred to as the headway – that is, the distance between two trains on the track - in designing track circuits.

Study the diagram below. Do you see how the signal pattern changes depending on what blocks are occupied in order to maintain a minimum distance between the trains?

This is why the maintenance of track circuits is so important. They keep trains moving on time, and most importantly, keep everyone safe.

But how do track circuits work?

Well, the rails connect a power source at one end of the block to a relay at the other end – creating a complete circuit. Low voltage power then flows down one rail, through the relay, and returns to the power source via the other rail. 

There are two variants to DC type track circuits. One runs of a traditional mains power source and the other runs off solar panels and batteries.

Over the course of the next few weeks, we’ll learn about the main failure characteristics of both solar panel and battery-powered DC track circuits. In week three, you’ll work through a series of common scenarios that might occur on a level crossing loop, and wrap it up in week 4 with a video pitch to your mentors, talking us through the issues you encountered and the strategies you used to solve them. 

For our first task though, let's have a look at building our own electrical circuit to understand more about how these signalling features work!