An investigation has been launched after a train travelled at 45mph in a 5mph emergency restriction zone put in place due to embankment problems.
The incident involved a Glasgow-London passenger service near Beattock at about 12:25 on 20 December.
The Rail Accident Investigation Branch (RAIB) said the restriction was in place due to an embankment failure.
It said there had been "no adverse consequences" but has begun an initial examination of the circumstances.
A safety digest will be published in the next few weeks based on evidence gathered to date.
A spokeswoman for Avanti West Coast, which ran the service involved, said: "We are co-operating fully with RAIB as they carry out their inquiries."