The driver or companies involved in the Croydon tram crash will not face criminal charges, police said.
British Transport Police (BTP) insisted that “every scrap of possible evidence has been scrutinised” after seven people were killed and 51 others injured when a tram derailed in south-east London on November 9 2016.
It came off the tracks at almost four times the speed limit in darkness and heavy rain.
The driver, Alfred Dorris, of Beckenham, south-east London, was arrested at the scene and questioned on suspicion of manslaughter.
BTP said he will not be charged with gross negligence manslaughter, and no charges for corporate manslaughter will be brought against Transport for London (TfL) or operator Tram Operations Ltd, a subsidiary of FirstGroup.
A report by the Rail Accident Investigation Branch published in December 2017 found it is “probable” Mr Dorris “temporarily lost awareness” on a straight section of track and may have fallen into a “microsleep” for up to 49 seconds before speeding round a sharp bend at Sandilands at 45mph.
BTP said its three-year inquiry involved forensic work at the scene, analysis of phone records, hundreds of interviews, and thousands of other individual pieces of evidence.
Detective Superintendent Gary Richardson, who led the police investigation, said: “We know that this latest update may not be the news that many, including the family members who lost loved ones, had hoped for.
“But we are satisfied that every scrap of possible evidence has been scrutinised and, after lengthy consultation with the CPS, it has been concluded that the threshold to bring charges of manslaughter against the tram driver, TfL and Tram Operations Ltd have not been met.”
He added that BTP will continue to work with the Office of Rail and Road as it investigates whether health and safety laws were breached during the incident.
The force will begin to work with HM Coroner to prepare for the victims’ inquests.
Jenny Hopkins, head of the special crime and counter terrorism division at the Crown Prosecution Service (CPS), said it had “carefully reviewed all the available material” before concluding that the evidence “did not support a prosecution”.
The CPS added in a statement that there was “no compelling evidence that the driver had done anything which he ought to have known could adversely affect his concentration”.
Law firm Osbornes Law, representing Andrzej Rynkiewicz, whose wife Dorota died in the crash, described the decision as “devastating for many of the families”.
Ben Posford, a partner at the firm, said: “Understandably the police wished to explore every avenue and gather potential evidence, but for the bereaved families the process has meant an interminable wait for answers as to what happened to their loved ones and why.”
He called for an inquest into the deaths to be “an open and transparent process” to explore the “systemic failures that led to this tragic event”.
TfL said it has implemented a number of safety measures since the crash, including a permanent speed reduction, better signage at sharp bends and a device that detects when a driver is distracted or fatigued.