A Greater Anglia train travelled for 23 minutes with one of its doors open, accident investigators have said.
The incident happened on a 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 last month.
The Rail Accident Investigation Branch said the door was open for about 23 minutes while the train travelled 16 miles at speeds of up to 80mph.
Martin Moran, Greater Anglia 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.”