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".
Read more on this story and others from London
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.

The attempted abduction happened at about 08:00 GMT while the boy was walking to school along North Road, Bourne.
The offender is described as 6ft and white, with short spiked brown hair and tattoo on his neck.
Officers are warning the public to not let children go out alone.
Det Insp Pete Grayson, from Lincolnshire Police, said the boy was very frightened by the incident.
He urged anyone with information to contact the force.

According to reports, he was a member of the firm's UK team.
He was understood to have been attending the Cannes Lions event, an annual gathering of people in the advertising and marketing industry.
In a statement, Google said: "We lost a loved and respected member of our team. We are deeply saddened and our thoughts are with his family and friends."
No further details about the employee have been made public out of respect for his family.

The pair collided while attempting to take a catch during Sunday's T20 Blast game with Sussex at Arundel.
The match was abandoned after ambulances took them from the field.
Australia international Henriques, 28, broke his jaw in three places, while Burns, 24, had cuts around his left eye that required stitches.
Surrey announced on Twitter on Monday that the pair were "in decent shape".
And a further update from the club on Twitter 24 hours later read: "More good news on the lads - Moises has now been released from hospital and will see a dental specialist tomorrow. He's improving slowly!"
Burns, meanwhile, tweeted his thanks "to all the medical staff" who treated the pair and his hope that his club colleague "keeps progressing well."
Team-mate Kevin Pietersen, who was in the field at the time of the collision, said the incident deeply affected the team.
"I kept thinking one of the guys will call. Then you think one will stop but then bang, bang. It was absolutely sickening," Pietersen told the Daily Telegraph.
"When those guys went down and did not move it hit some of our guys very hard.
"It was the right decision to abandon the match. We had lost two of our players, a lot of players were crying in the dressing room and it is only a game of cricket."
Director of cricket Alec Stewart also posted a picture on Twitter of Burns smiling in his hospital bed.
Henriques, who has played three Tests, six one-day internationals and four T20 games for Australia, joined Surrey in April as an overseas player for their T20 Blast campaign.
Surrey-born Burns, a left-handed batsman and occasional wicketkeeper, made his first-class debut for the county in 2012.
Surrey's County Championship game with Glamorgan began as scheduled on Monday.
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.
[NEXT_CONCEPT]
Police are hunting a man who tried to force an 11-year-old boy into the back of a black van in a Lincolnshire market town.
[NEXT_CONCEPT]
A Google executive has been killed in an accident in Cannes, France, the search company has confirmed.
[NEXT_CONCEPT]
Moises Henriques has been released from hospital following the on-field collision with Surrey team-mate Rory Burns that left him with a broken jaw.