A passenger train driver broke the speed limit in a 20mph zone - by nearly 100mph, an investigation has found.
Emergency restrictions were in place near Dauntsey, Wilts., on the day of the incident due to hot weather and a track defect.
But the driver of the London to Bristol service had "no means of knowing" this, a probe has ruled - causing the speeding.
A limit of 20mph had been imposed for both freight and passenger trains on August 12 because of the problems.
The line speed in the area is usually 125mph but had been reduced to 30mph for freight trains on June 27, before the additional restriction was later imposed.
But the unknowing driver continued at 117mph as Network Rail had not informed train operator GWR about the change, the investigation said.
The driver reduced his speed to 105mph when he realised the mistake before clearing the section of the line where the limit was in place.
No one was injured and there was no damage as a result of the incident.
The Rail Accident Investigation Branch (RAIB) said lineside information "did not sufficiently highlight" that existing restrictions "had been modified".
It decided not to carry out any further investigations into the incident, but that it showed a "warning" of a restriction ahead "would be valuable".
The ruling stated: "This incident is an example of circumstances in which a warning of an emergency speed restriction (ESR) ahead would be valuable.
"The driver was misled by the way the lineside information was presented, which did not sufficiently highlight that the existing ESR had been modified such that passenger trains were now affected."
