Network Rail have described it as the train going through a red signal and causing an “Automated Derailment”, which to the uninitated does sound like an odd turn of phrase.

It actually derailed on a trap point, which is designed to protect the exit from a siding when the main line isn’t clear for a train to leave the siding. A low speed derailment such as happened outside Paddington would be far less dangerous than a collision with another train.

The above film is from The Great Central Railway back in 2013, and shows a trap point doing what it’s supposed to so. It’s on a heritage railway using traditional mechanical signalling, but the principle is just the same.

Notice that the signal is at danger, and stays at danger. According to the comments the derailment you see happen was the result of a signaller error; the signaller gave the driver an instruction to proceed past the signal even though it was at danger, but hadn’t set the points correctly.

Red faces all around. Fortunately nobody was hurt, and the damage was repairable.