A Croydon tram which crashed this morning, killing five people, was travelling at a significantly higher speed than is permitted, says the RAIB.
Initial investigations by the Rail Accident Investigation Branch suggest that the tram approached the steep curve where it crashed above the 12mph speed limit.
An RAIB spokesperson said: “The derailment occurred on the curve and initial indications suggest that the tram was travelling at a significantly higher speed than is permitted.
“We are currently collecting evidence needed to identify factors relevant to the cause of the accident and its consequences.”
The incident, which occurred just after 6am this morning, also injured 51 people.
The driver of the tram was arrested after the incident.