The Croydon tram crash could have been prevented if more action was taken after a similar speeding incident days earlier, the widow of a victim has claimed.
Seven people were killed when a tram derailed on a sharp bend just after 6am on November 9 last year, with a further 58 injured.
Marilyn Logan, whose husband Phil died in the crash, said she is "heartbroken" that an incident on October 31 was not acted upon quickly enough.
The final report by the Rail Accident Investigation Branch (RAIB) will be published on Thursday.
Mrs Logan, who has seen a copy, said she believes the crash could have been avoided due to the earlier incident.
A passenger raised concerns about the speed of a tram at the same location as the fatal crash but there was a "culture of fear" among staff which meant drivers were not reporting incidents, she said.
"From what I've read, the driver went round so fast that when the tram got to Sandilands the lady got off the tram because she really thought it was going over," Mrs Logan told the BBC.
"Now, had that incident been investigated, the one a week later might never have happened."
An interim report by the RAIB stated that the two-carriage tram involved in the crash was travelling at 46mph as it entered a sharp bend at Sandilands Junction, which had a 13mph limit.
It came off the tracks, overturned and slid for 25 metres.
The late application of the brakes, and the absence of emergency braking, suggested tram driver Alfred Dorris "lost awareness".
The 44-year-old from Beckenham, south-east London, was arrested at the scene and questioned on suspicion of manslaughter.
He was last questioned by police in September, when he was again released on bail.