A probe has been launched by Greater Anglia and government safety staff to find out how a train travelled through Essex for 23 minutes with a door open.
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 on August 22.
The incident happened on one of the refurbished electric suburban trains that are used on services across Essex and up to Ipswich - but which are due to be replaced by new trains by the end of next year.
The Rail Accident Investigation Branch 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."
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