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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
If you enjoyed our #geekthread on the upcoming #HitherGreen engineering work, which marks the end of 46-years of signalling from this brutalist building at London Bridge, you might enjoy this mini-thread... /1
Among the sadness from everyone who has worked at this signalling centre is the knowledge that the railway is always evolving, and when London bridge box opened it took with it some smaller boxes, including the busiest in the country...
Borough Market Jn (pic @RailwayMuseum ). /2
Borough Market Jn (pic @RailwayMuseum ). /2
Perched high on the viaduct above the market, it signalled 100 trains per hour at the busiest times, using an old lever frame (pictured), and was a two-man job. Amazingly, the box has been preserved @RailwayMuseum , which reopens to the public on 4 August. /3
