It emerged that Miss Banks had earlier been seen at Tunbridge Wells Hospital but was sent away as she was not in labour and contractions had not started.
Commuter Neil Perkins, 22, a recruitment consultant from Maidstone, was sitting close to Miss Banks before she went into labour. He said his "jaw dropped" when he realised she had given birth.
The father of a baby girl born on a train packed full of commuters during rush hour has spoken of his joy.
Allan Stanley was travelling back to his mother's home with her and his partner Sonia Banks when the contractions started.
The grandmother of a baby girl born on a train packed full of commuters during rush-hour said today: "We're all still in shock."
Mari-anne Stanley was travelling home with the unnamed woman who suddenly went into labour on board the 18:18 London Victoria to Maidstone East service last night.
Passengers on board the Southeastern service were asked to move off the train or into a separate carriage as a midwife, nurse and police officer, who were off-duty, came forward to help.
Ms Stanley said the mother gave birth a week early to a "perfectly healthy" 6lb 13oz baby called Phoebe on board - who was greeted with cheers by other passengers.
A man claiming to be the uncle of baby Phoebe, who was born on a commuter train from London Victoria to Kent yesterday evening, has tweeted to say his niece is "well and safe."
A baby girl has been born on a train packed full of commuters. The Southeastern train was delayed at West Malling in Kent while the infant was delivered on board the 18:18 London Victoria to Maidstone East service.
A spokesman for the company said he did not know who helped deliver the child.He said: "The baby is doing OK and has been named Phoebe. The normal protocol for a medical emergency is for the conductor to administer first aid but it is likely he would have had to put a message out to ask for help."
Passengers took to Twitter to offer congratulations. @thumper--1111, said: "Baby girl just been born on 18:18 Victoria to ashford at west malling. marvelous grats to parents hope all well."But another commuter said: "That's the first good excuse Southeastern have come up with for a delayed train."