Passengers were thrown from their seats and hit by falling luggage on a train being driven at 50mph above the speed limit, investigators said.
The Lumo service passed over three sets of points near Peterborough station in Cambridgeshire at 75mph at 10.20am on Sunday April 17, the Rail Accident Investigation Branch (RAIB) said.
The maximum permitted speed for the points – which are movable sections of track allowing trains to switch from one line to another – was just 25mph.
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At the time of the incident, British Transport Police said 15 passengers reported minor injuries and were offered medical assistance by paramedics.
The force received a report that the train’s emergency brake was applied.
In a preliminary statement, the RAIB said: “Passing over the points at this speed (75mph) meant that the train suddenly lurched sideways.
“The sudden movement of the train resulted in some passengers being thrown from their seats and in luggage falling from the overhead storage in the passenger compartments.
“This resulted in a number of minor injuries being sustained by passengers.”
The service involved was the 8.20am departure from Newcastle to London King’s Cross.
It came to a stop after passing through Peterborough.
The train was not due to call at the station.
It initially approached the area on a fast line, before being routed on to a slower line via the points where the speed limit was broken.
The RAIB is carrying out a full investigation into what happened.
This will include consideration of the way the train was driven, any factors which may have influenced the driver, the condition of the signalling system, the nature of the injuries, and any underlying management issues.
A Lumo spokeswoman said: “At Lumo, the safety of our customers and staff is our highest priority, and we will be co-operating fully with the RAIB’s investigation.
“Until the investigation has concluded it would not be appropriate to comment further.
“In the meantime, we remain committed to providing the highest standards of service to our customers.”
Lumo, owned by FirstGroup, began running services on the East Coast Main Line between London King’s Cross and Edinburgh Waverley in October 2021, competing against state-run London North Eastern Railway.
It is an open access operator, meaning it does not receive Government funding.