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 76mph.
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.”
