At least seven people were killed when a tram sped round a sharp bend and derailed during the morning rush-hour.
Investigators said the vehicle was travelling at a "significantly higher speed than is permitted", and are probing whether the driver, who has been arrested, may have fallen asleep.
Scenes on board have been described as "total carnage" and "like something out of a film" as the two-carriage tram tipped onto its side next to an underpass near the Sandilands stop in Croydon, south London.
The 42-year-old driver, from Beckenham, is being held on suspicion of manslaughter and is currently in police custody, British Transport Police said.
One passenger, who was among more than 50 injured, recalled being trapped under a woman, adding: "I don't think she made it".
Martin Bamford, 30, from Croydon, added that the driver told him he thought he had "blacked out".
London mayor Sadiq Khan, who visited the scene earlier, warned the death toll "may well increase".
The tram was operating from New Addington to Wimbledon via Croydon when the accident happened at 6.10am on Wednesday.
British Transport Police's assistant chief constable Robin Smith said they were investigating whether the driver of the Wimbledon-bound tram fell asleep at the wheel, alongside "a number of factors".
Initial findings of the Rail Accident Investigation Branch (RAIB) show that the tram came off the tracks as it was negotiating a "sharp, left-hand curve" with a speed limit of 12mph.
An RAIB spokesman said: "Initial indications suggest that the tram was travelling at a significantly higher speed than is permitted."