The 27-year-old was getting off the Southeastern train at West Wickham, Bromley, when the doors shut on her.
The driver did not see the woman on CCTV and moved off, dragging her along. She then fell under the train.
Southeastern said it had offered support to the woman after the "terrible incident" in April 2015.
The report was carried out by the Rail Accident Investigation Branch (RAIB).
A Southeastern spokesman said the company had offered to help the woman "as much as possible since the accident".
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The woman had tried to free the stuck bag strap but she was pulled along when the train moved and fell underneath it, resulting in "life-changing injuries", the report said.
Investigators found she was able to open the doors two seconds after the driver had begun the door closure procedure, but was given no indication that they were about to shut.
The RAIB recommended all operators who have trains which use power operated doors should make modifications to ensure passengers always had a safe amount of time to use them.
They also said drivers should watch train doors while they are closing, as well as checking all doors are shut correctly.
Neither the driver, who was a trainee, or their supervisor saw the woman on platform monitors and only found out about the accident later.
Southeastern said it had "worked very closely" with the RAIB and has "complied with all the recommendations and learning points they have made".
A spokesman said this included altering the doors on Southeastern trains to ensure they always remain open for a safe period of time.
A woman was seriously injured when her bag got caught in a train door, dragging her along the platform and on to the track, a report has revealed.