A runaway railway trolley hurtled along a track for more than half a mile (0.8km), narrowly avoiding a group of rail workers.
The accident happened near the Cheshire village of North Rode at about 05:00 BST on 26 May.
The trolley ran downhill towards a team of track workers and smashed into equipment, although nobody was injured in the crash.
The Rail Accident Investigation Branch (RAIB) said it was looking at how the crash happened, including whether the design or maintenance of the trolley contributed.
‘Brakes failed’
The investigation will also examine whether workers did anything wrong, or whether “underlying management factors” played a role.
The RAIB said the trolley was being used to transport work equipment when its brakes failed.
It then rolled away from the team that had been using it for 1,100m before the crash, which caused it to derail and damage the equipment it struck.
The RAIB probe is independent of any investigation by the railway industry or by the industry’s regulator, the Office of Rail and Road.