A passenger train travelled at 80mph with one of its doors open due to loose screws, accident investigators have found.
The Greater Anglia train covered 16 miles through Essex before a passenger alerted the driver.
A report by the Rail Accident Investigation Branch said that a door was open for 23 minutes during the journey on August 22.
Investigators found the door interlock system - designed to detect if any doors are not fully closed - failed because two door bracket screws had become loose and fallen out.
This meant the train was able to move away when commanded by the driver.There was no indication that the screws had been tightened during refurbishment of the train.
Following the incident, Greater Anglia found loose screws in at least 60 out of 480 doorways on its refurbished Class 321 trains. The RAIB said the screws should have been tightened.
We apologise to customers affected by this incident. We take the safety of our trains and customers extremely seriously. We have cooperated fully with RAIB in their investigations as well as carrying out our own investigation. We have introduced new, more stringent safety procedures and are committed to carrying out additional checks on our trains' doors from now on.
Safety on our railways must always be paramount. Passengers need to have complete confidence that they will not be put in danger whilst riding a train.