Thread by @NetworkRailSE

We’ve been filling your timelines with #HitherGreen content this week to show what we’ve been doing while no @Se_Railway trains are running in the area, and now it’s time for a serious #geekthread! What is signalling and why are we replacing it? /1

Signalling allows for the safe movement of trains, making sure they don’t hit each other, and making sure they’re literally on the right tracks. A train travelling at 90mph will take a mile or so to stop, so these are much more than traffic lights. /2

Our 92 signals in #HitherGreen are now mostly 4-aspect (colour) LEDs. The reason for 4 aspects is to give trains time to slow down. These go red, yellow, double yellow, green. Red means STOP, yellow means “caution, the next signal is red”. /3

The double yellow (like in this old, pre-LED signal) gives trains extra time to slow down and means we can run trains more closely together – really important in this area where there’s so many trains about. /4

Each signal will have trackside equipment associated with it to give train drivers audio warnings of what aspect (stop or go) the signal is showing. This is old kit being removed on Saturday. /5

In most cases the Train Protection + Warning System, which looks like this, will also stop the train if it thinks the driver hasn’t slowed down enough to stop at a red or if the train is going too fast for the track. It’s a hugely important safety feature. /6

Points are also part of the signalling system, as these allow trains to cross from one track to another. They’re sensitive bits of kit, so we’re enhancing 86 sets at #HitherGreen to improve reliability. This is part of a set being lifted in to place. /7

Points work by literally moving two pieces of rail so the train wheels follow them (this pic is at Elgin, shot long before COVID!). Our system needs to know the rails are locked in place before it will allow trains to cross them. You know “points failures”? /8

That’s usually when we have “lost detection” on the points and we can’t tell if they are locked safely in place. Most of our points now have remote monitoring so we can get in and fix them before they go wrong. This is a set with the covers off! /9

But what about controlling it all? Signallers see trains as 4-digit codes moving across their displays and the final aspect of signalling that underpins everything is train detection… or how we know where they are! /10

The railway is divided into sections where only 1 train can be. The system knows if there’s a train there thanks to two forms of technology – track circuits and axle counters. /11

Track circuits are low-voltage electric currents fed through the rails. A train’s metal wheels short circuit the current… And the system knows there’s a train! The ones near London Bridge are more hi-tech, using audio frequencies, but that’s enough for this thread. /12

Axle counters are much simpler, they use detectors to spot changes in magnetic fields when a train wheel passes by to count the wheels on a train. This close up is one of the detectors. /13

We used to have 90 track circuits at #HitherGreen but they were old and unreliable and caused us (and you) lots of headaches. We’ve now got 262 Frauscher axle counters instead as part of the new system, which count wheels in and out of each section. /14

To grossly simplify, we put track circuits in areas where there are lots of very short sections and complex junctions, and axle counters where it’s a bit less intense. Hence circuits at London Bridge, below, and axle counters at #HitherGreen (and also much of East Kent). /15

Finally, the other major change at #HitherGreen is the transfer of control from London Bridge signalling centre to Three Bridges operating centre in Sussex. Essentially, London Bridge was a massive analogue computer... /16

…with a room full of relays (switches) like this, providing the logic that kept trains safe. Put simply, the system was designed so that if signal A was green, signal B must be red and vice versa. It’s binary Jim, but not as we know it. /17

Signallers stood at panel 7 and pressed buttons to send a train from signal to signal at Hither Green, and the relays clacked away downstairs to make sure it was okay to do that. No more! /18

Three Bridges uses computers to do the same job, and Hither Green is now controlled through this WestCAD set up. For security reasons, it’s a private network and each area can ONLY be controlled from that workstation. /19

This map (Hello MS Paint) shows the control panels that now look after South London – VIC is Victoria signalling centre at Clapham, Cannon St, Charing Cross, Grove Pk +Lewisham workstations are at Three Bridges, and Ashford speaks for itself. /20

We are gradually moving over to fully-automatic route setting, so signallers will only need to intervene in the running of trains if something goes wrong or when there are delays and a train needs to be rerouted or timed. This pic is the West of Scotland signalling centre. /21

If that sounds easy, you’re wrong. When something does go wrong, all of sudden the lives of thousands of people are in the hands of our signallers and @Se_Railway drivers, and they have to know what to do and do it calmly. /22

So, it’s a complicated system that only works thanks to the people who install it, maintain it and run it. If you think you can be one of those people, get in touch – it’s a great industry and never dull…!
Normal service resumes at #HitherGreen on Monday.
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