The tram driver, arrested on suspicion of manslaughter, was reportedly asleep at the wheel.
Witnesses recounted the “carnage” with reports that the driver, 42, may have blacked out before the two-car tram overturned near a tunnel in Croydon about 6AM as torrential rain battered London.
The tram derailed when rounding a sharp curve with a 20kph speed limit on the approach to a junction as it traveled from new Addington to Wimbledon.
All the 51 injured people have been sent to the hospital.
The rail accident investigation branch says that the tram was traveling at a “significantly higher speed than is permitted.”
The traffic has been suspended at the site of the accident, and the TFL said that it will not resume until the weekend at least.
Police officers are continuing to work at the scene. They will carry out forensic examinations overnight and on Thursday assess how and when it is appropriate to recover the tram and remove it from the tracks.