Alsi the DLR uses a fail-safe signalling system that was designed and tested for that purpose.And I am assuming you've noticed it is totally self contained without any other conflicting traffic, fairly small, has slow speeds, uses the same rolling stock used everywhere on the system and doesn't have traffic or pedestrian crossings. Very very different to the national network.
As the reply above said, I don't think anyone is saying it is not possible. But it is probably further off than some people here want to think, and holding up something like the DLR as "proof" it could happen quickly isn't proof at all.
Not some shonky snake oil whose current killer application is to add pointy ears and boggly eyes to a photo of your face.