Yesterday, Great Eastern, which operates the rail line between London and Southend, Essex, launched an inquiry into the incident.
Susan Kelly, 33, an insulin-dependent diabetic who needs four injections a day, said she could have died after she fell into a coma during her journey home and the train was shunted into a deserted siding.
It wasn't until the train was taken back into the station to be cleaned four hours later that she was able to open the sliding doors and stagger out.
"When I came to, I was drifting in and out of consciousness," said Miss Kelly, an accountant from Leigh-on-Sea. "I was aware of other trains passing by but I couldn't raise the alarm or make contact with anyone. At first, I wasn't scared but then the seriousness of my plight began to sink in and I realised I could die. I have to travel on trains to get to and from work but now I find it a traumatic experience.
"Staff should check all carriages before they are locked - it's one thing if someone is drunk and asleep but it is a far more serious if someone had fallen ill."
It was Miss Kelly's father, Brian, a retired businessman, who raised the alarm after she failed to turn up for dinner. "We checked with the station and the rail operators and even the transport police," said Mr Kelly. "But there had been no delays and no one had been taken ill on a train.
"Susan has lived with diabetes for more than 20 years and she has developed strategies to cope. She wears a Medi-alert bracelet, carries a diabetes card and always has insulin and sweets with her for emergencies." But despite all these precautions it wasn't until four hours later that she was found by the cleaners. "Other passengers may have thought she was asleep or drunk but a simple common sense check would have shown that she was in a coma," said Mr Kelly.
Miss Kelly's ordeal happened on 11 August after she got onto the 5.45pm train from London's Liverpool Street to Southend Victoria station, where she parks her car every morning before catching the train to London. "We checked and found her car there and that is when we really began to worry," added Mr Kelly.
"The train would have got there around 7pm but was 10.45pm before it was moved back into the station for cleaning." Mr Kelly said he had not had a reply from the rail operator despite making a complaint the day after the incident.
Great Eastern yesterday gave Miss Kelly a full apology and announced an investigation. "Trains are usually checked by staff before they are put into sidings because people do sometimes fall asleep on their journey and miss their station," said the company's spokesman. "We are trying to find out if it was done on this occasion. Our processes are now being reviewed and the matter is being investigated at director level because we do take our responsibility seriously."
A spokesman for the British Diabetic Association, said some people had symptoms before they went into a hypoglycaemic coma, like sweating or seeming disorientated. "But if she did not have the symptoms, it may well have looked like she was just falling asleep."