Loose screws were to blame for a door being open on a passenger train going at 80mph (128km/h), a report found.
The train travelled 16 miles through Essex once the screws fell out, before a passenger alerted the driver.
A Rail Accident Investigation Board (RAIB) report said operator Greater Anglia had since found loose screws on “at least 60 doorways” in its fleet of refurbished class 321 carriages.
The company said it had improved safety procedures and brought in extra checks.
The screws fixing the door bracket to the closing mechanism showed no signs of having been tightened properly during refurbishment, said the report.
The missing screws meant that the door mechanism, unattached to the door, was able to return to the closed position – giving the driver the incorrect signal that all doors were shut.
The door was open for 23 minutes until it was reported at Hockley station, Essex, at 07:20 BST on 22 August, and the screws and washers were found by the driver.
The RAIB said the screws in the refurbished units should have been tightened and a yellow line drawn across the screw head and bracket, so that any loosening would be recognised – but no marks were found.
This meant “one of the visual indications of a loose screw was absent, reducing the likelihood of checks identifying any loosening”, said the RAIB.
Greater Anglia has since retightened all door screws on its refurbished carriages to the correct torque after discussions with the parts manufacturer.
A spokesperson for Greater Anglia apologised to passengers, and said it took safety “extremely seriously”.
“We have co-operated fully with RAIB in their investigations as well as carrying out our own investigation,” she said.
“We have introduced new more stringent safety procedures and are committed to carrying out additional checks on our trains’ doors from now on.”