⚠️⚡️ Were so sorry for the disruption to @Se_Railway + @TLRailUK passengers on the Orpington/Charing Cross line this morning - this is the problem right here, at #PettsWood. It’s a “blown insulated block joint”, although actually two of them have gone. /1 
You can see the blocked lines just outside #PettsWood as they have BLOC in their train indicators. We’re able to run on the other 2 lines and the connecting loops at the bottom of the picture, which is why there is still a train service, albeit not a normal one. /2
An insulated block joint is used to separate 2 track circuits, which use electric currents in the rails to tell us where trains are. This is one in happier times. /3
The rails are also used to send return current from trains back to earth via sub-stations along the railway (completing the circuit between live rail/train/earth like a simple battery and bulb circuit you might have built at school). /4
We use special connectors (called impedance bonds) to pass that current around the block joints, but if anything goes wrong with the joints or the connectors, the return current tries to jump the insulated block joint and ⚡️🔥 results. /5
Bear in mind our trains run on 750v and you can see why this arcing might make a mess like this. We’ve cut the damaged joints out now and we’re putting new ones in, before our signalling techs will set up and test the circuit. /6
We hope to have all this finished by mid-morning, but we realise that doesn’t help people travelling to work, so please accept our apologies for the disruption today. Ends.
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