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.