Fail safe systems are all around us in our every day lives. They're a staple of safe engineering.
Think of elevators – they're designed with special brakes that engage if the cable snaps or loses tension. Likewise, many machines will completely shut down if they detect that something's wrong.
Well, the same applies to signalling! When there’s a fault, all signals will default back to red 🔴 to indicate stop and all level crossings will drop ❌ In rare cases where the signal bulbs don’t work at all (for example, where the bulb filament itself is the issue) the principle “signal off is equal to signal red” applies. That way, even when the signal is switched off, a driver will always know to stop, just to be on the safe side.
But how do signals know when to display what?
Signalling 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.
Last week, we considered how modern ATMS signalling will help reduce the headway between trains. Until then, though, most train systems in Australia still operate using what’s called ‘track circuits’, which basically use electric currents to detect when a train enters, occupies, and leaves a ‘block’ or section of track. The train's passage over the track circuits triggers a lights system a lot like your everyday red, yellow, and green traffic light.
Study the diagram below. Do you see how the pattern changes depending on what blocks are occupied, but the signalling system always maintains a minimum distance between the trains?
Aside from the signalling design itself though, it's also important to consider where the equipment and tracks themselves are going to be laid out – for example, what if the signalling equipment needs to go in a bushfire or flood prone area? How do we ensure that everything stays operational when disaster strikes, so that you as the Operator can coordinate train movements and get everyone out of harm's way?