Network Rail Kent and Sussex

Each location had to be manually reset, which took us until 5.47pm, when our signalling was fully restored. We then worked with our colleagues to get trains moving again and try to get a stable service plan in place to get everyone home from London. As you saw, this took time. /3

We can only apologise for what was a terrible evening for our passengers, who took hours to go home and in some cases missed flights from Gatwick Airport. Many of you asked us how this could happen - so this is what we know. /4

The power supply to our signalling in this area had a power surge, what is known as an Over Volt, for 20 seconds. This is an unusually long time for that kind of surge and many different systems - some only 18 months old - all shut down to protect themselves. /5

Our technicians went to four different locations across the Croydon area and manually reset them, which fixed the problem and allowed trains to run. Had those systems not shut down they would all have been damaged by the surge. /7

In fact it is unlikely we would have been able to run a service today either as every location would have needed to be repaired or replaced. So the systems worked as designed and saved us from a much worse fate. /8

So why didn't the backup kick in? We have three separate power supplies to our signalling to stop this from happening, + they are designed to switch over when the power is cut. But the power stayed on, just at the wrong Voltage. /9

Why don't we have Uninterruptable Power Supplies? Well, we do at some locations, such as signalling centres. What we are not sure of - and are investigating - is if a UPS at each location would have stopped this or if the voltage spike would have put them offline too. /10

A UPS at a nearby location actually put itself into "bypass" during the spike, which suggests it is not a clear-cut answer. Another important part of the investigation is why we cannot remotely reset signalling control points, and how quickly that can be rectified. /11

So what is next? We are working with our operator colleagues to investigate the disruption, with @UKPowerNetworks to learn more about the power surge + our many signalling suppliers about how we can make our kit more resilient to this kind of event. /12

Finally, we can only apologise for the disruption last night, as we know what a huge impact it had on everybody. When the full investigation is complete, we will let everyone know what it finds (beyond what we know so far) and how we will make things better in future. /13