A train driver killed in a crash in Bedfordshire passed a red signal moments before the collision, investigators said.
Train driver Shaun Burton, 60, died and about 100 people were injured when one London-bound service crashed into the back of another on Friday.
The Rail Accident Investigation Branch (RAIB) said in an interim report that Burton’s train proceeded past a red signal near the scene of the crash in Elstow, near Bedford at 17:15 BST on Friday.
It added that “it is not yet possible to say what indication the driver received” from automatic warning system (AWS) equipment fitted to the train.
The RAIB has found the stationary train had come to a stop “unexpectedly” because of a fault with its Automatic Warning System (AWS) equipment.
Data suggests the signal behind the stationary train was red, according to investigators.
The train that had started its journey at Corby, passed this red signal, its brakes were activated for about nine seconds before the collision, when the train was travelling at about 76 mph.
Its speed had reduced to 49mph when the impact happened.
The RAIB said its full investigation would consider “the actions of those involved and any factors that may have influenced them”.
Dave Calfe, the general secretary of train driver union ASLEF, welcomed the investigation and said the incident may have been avoided if the train protection and warning system (TPWS) had been installed in that location.
He said: “Because what will concern passengers, and what concerns us, is that there are safety systems that can be put in place to prevent such accidents happening and the network should have been able to cope.”