The incident happened on a Greater Anglia service from London Liverpool Street to Southend Victoria.
A passenger alerted the driver that the door was open on the side opposite the platform at Hockley station in Essex at around 7.20am on August 22.
The Rail Accident Investigation Branch (RAIB) said the door was open for about 23 minutes while the train travelled 16 miles at speeds of up to 80mph.
It conducted a preliminary examination and will publish a document in the coming weeks highlighting what safety lessons can be learnt.
Greater Anglia commercial and customer service director Martin Moran said: "Safety is our highest priority. We immediately took the train out of service when this happened and carried out our own investigation into the incident.
"We have also carried out checks on every single door on that type of train that we have.
"No-one was injured in this incident and there have been no further incidents since."
A separate RAIB report published in July found that a Tube driver's lack of sleep and food may have contributed to 10 doorways being open as a Jubilee line train travelled between two stations.
It made four recommendations to London Underground, including improvements to door control systems, better training for drivers and raising awareness of how insufficient sleep and inappropriate eating patterns affect safety.