As it passed through Pollhill Tunnel at 60 mph the rocking became violent and the train derailed past Dunton Green railway station, where the line is in a cutting which is spanned by a bridge carrying Shoreham Lane.

John Wallace Pringle, Chief Inspecting Officer of His Majesty's Railway Inspectorate conducted the inquiry in person.

[1] Following the accident, the Southern Railway withdrew all the remaining 'River' class tank engines, which caused a public sensation at the time.

Other drivers testified about the instability of the class and it emerged that one locomotive had previously derailed at speed, though it had re-railed itself.

Trials carried out after the accident showed that the design behaved well when running at 85 mph on the Great Northern main line out of King's Cross, indicating that an indifferent permanent way was partly responsible for the instability of the locomotives.