The District Line train had been brought to a halt, after the driver passed through a red signal. After a few minutes he was given permission to leave & the Piccadilly line train crossed the junction immediately ahead.
The driver was obviously shaken but it's thought that the delay after the red light saved the trains from crashing. The Piccadilly Line was travelling at 25mph and both trains would have been derailed if they'd hit.
A spokeswoman for TfL said "Safety is London Underground's top priority and we have an excellent safety record." The incident has now been reported to the Rail Accident Investigation Board and the Railway Inspectorate. See Current.com / Metro for more.
The investigation will hopefully reveal what happened, but I still don't understand how trains are actually physically able to pass red signals in the first place. I'm sure someone can technically tell me how this will happen, but I remember being shocked that SPAD's or "Signals Passed at Danger" (the Tube equivalent of going through a red light) could take place. I'm assuming this doesn't happen on the DLR where the trains are all fully automated. It seems to be by luck & driver ability rather than the design of the system that more serious accidents haven't taken place.