The “trap and drag” incident took place at Ealing Broadway station just after midnight on November 24 last year.
As the train doors closed, the hand of a passenger trying to board the train became trapped.
They were then forced to run alongside the carriage for several metres as the train pulled away from the station.
Fortunately, they were pulled to safety by a railway worker who was on the platform at the time. The passenger sustained minor injuries.
The driver stopped the train, which travelled 17 metres, after being alerted by a nearby passenger to what was happening.
The Rail Accident Investigation Branch (RAIB) announced last week that it would be looking into the incident.
Its report is expected to look at the actions of those involved, the measures in place to manage boarding risks on Elizabeth line trains, post-incident management and any underlying management factors.
A watchdog spokesperson added: “We will publish our findings, including any recommendations to improve safety, at the conclusion of our investigation.”
A TfL spokesperson said that safety is its “top priority” and that operator MTR Elizabeth line is also carrying out an investigation.
They added: “We’re sorry that our customer experienced this distressing incident.
“We will always strive to learn from incidents and improve operational safety.
“MTR-Elizabeth line is fully co-operating with the RAIB to establish the root cause of this incident to determine what steps they can take to avoid any further events of this nature and to share learnings with the industry.
“While serious injuries on our network are rare, we are undertaking a huge range of work aimed eradicating such incidents and making travelling even safer for everyone.”