There are a million questions one could ask of the woman sitting on the edge of a hospital bed in southern Sri Lanka, but for now only one of them matters: is she Padma Wawlanbokke?

Five years ago, the 42-year-old's family mourned after she, her husband and the couple's young children disappeared and were presumed dead after the train in which they were travelling was struck by the deadly Asian tsunami.

An estimated 1,500 passengers and villagers lost their lives after the tsunami hurled the Matara Express from its tracks in the quiet coastal hamlet of Peraliya.

But just three weeks ago, Mrs Wawlanbokke's brother, Premawardan, was buying spare parts for his car in a town near his home when he spotted a woman begging beneath a clock-tower built during the days of British rule. Instantly, she reminded him of his sister. He returned with another sister, who was equally adamant that the dirty, unkempt woman with the outstretched hand was their sister, Padma.

Not all members of the family are convinced, and DNA tests are being carried out to determine whether the middle-aged woman truly is Padma. But if the results are positive then hers is perhaps the most remarkable survival story to have emerged from the tragedy of 26 December 2004, when two deadly waves tore across a swathe of south and south-east Asia.

Frustratingly, for now the grey-haired woman is of little help in determining who she really is and how she spent the last five years. When Mr Wawlanbokke took her to the clinic in the town of Galle she was deeply agitated and had not spoken a word. Gradually, however, she has started to speak.

When The Independent visited and showed her a picture of Padma Wawlanbokke taken from her wedding album and asked about the identity of a man in the photograph, a deep smile broke across her previously melancholy face and she replied without hesitation: "That is my husband."

Mrs Wawlanbokke and her family had boarded the ill-fated train from Colombo on the morning of 26 December to return to the southern city of Matara, close to their home.

The qualified nurse and her husband, a local government official, had been visiting a niece in Sri Lanka's capital, Colombo, with two of their sons, Ama, 6, and two-year-old Nitya. The couple had expressed an interest in buying a home there and relocating.

A series of painful ironies marked the destruction of the train as it travelled south, hugging Sri Lanka's western coastline. First of all, the express was named Samudradevi, or Queen of the Sea. Secondly, the train had been atypically on time.

Had it been just 10 minutes late, it would not have been at the exposed village of Peraliya, where the tracks are no more than a few hundred metres from the ocean. Had it been 10 minutes early, it would have been at the next station, in a town which suffered considerably less damage.

And had the train not withstood the tsunami's first wave, scores of terrified villagers may not have been tempted to rush aboard the already cramped and crowded carriages to escape the might of the sea. When a second, more forceful wave - perhaps reaching up to 40ft in height - hit the coastline, the train was lifted and thrown from its twisted tracks.

For several years, a number of the battered carriages were left in the town next to Peraliya, a striking if unnecessary reminder of the disaster.

Padma Wawlanbokke had told her family that she would be returning to Matara that day on board the morning express. "We knew that they were coming by train. And we heard on the TV and radio about the train accident," said Karunawathiee Wawlanbokke, 67, one of Padma's two elder sisters, who lives in a simple house in the jungle near the village of Beligalla.

"Afterwards, the niece and her husband travelled from Colombo to look for them. We did not find anybody." She added: "We said prayers for them. All this time we thought she had died as well."

The family said that Padma's brother had last month been in Ratnapura, known as a gem-mining centre, when he spotted a woman begging. Convinced it was Padma, he returned home and collected other family members, including another sister, Sumansaali.

They contacted the police who, they say, gave them permission to take the woman home. She was feisty and violent, smelled badly and would not let anyone touch her, said Mrs Wawlanbokke.

Up to 40,000 people in Sri Lanka were killed when the tsunami's waves struck with incredible power. Many were hastily buried, often in unmarked mass graves such as those dug on the coast outside Peraliya, where local people have erected a shrine.

Thousands remain unaccounted for and countless families may still harbour some flickering hope of a miracle; when news of the "discovery" of the woman in Ratnapura was reported in a local newspaper, a family from outside the district travelled to Mrs Wawlanbokke's home in the hope that the woman would turn out to be their missing relative.

They carried with them a leaflet that bore the photograph and details of a middle-aged, brown-haired woman. Mrs Wawlanbokke said she was only 50 per cent sure that the woman her brother found is Padma, whom she remembers as "very pretty, slim, handsome [and] happy".

"If it is my real sister, then I am happy," she added.

Other members of the family, notably Premawardan and Sumansaali, have no doubts. Sixty-five-year-old Sumansaali said: "I think it's her. There is a mark on her hand that is the same. Also her face."

But the members of Padma's husband's family are equally adamant that the woman found begging is not their in-law.

It was to them, according to Padma's direct relatives, that the official compensation of 60,000 Sri Lankan rupees (NZ$727), a considerable sum in rural Sri Lanka, was paid after the deaths.

Sitting in his house, Rohana Munasinghe, who is married to the sister of Padma's husband, listed a series of physical discrepancies between his sister-in-law and the woman found in Ratnapura that led him not to believe the story.

He said that his wife and daughter had twice visited the woman and had concluded she was not his relative. The women said that Padma had twice given birth by Caesarean section but that this woman bore no such scars. Her legs also appeared different, and there had been a patch of white hair on Padma's hair that did appear on the woman's.

"Padma could not stay on the streets for five years," said Mr Munasinghe's daughter.

"She is not strong enough."

As the family passed around several striking photographs of Padma taken on her wedding day, Mr Munasinghe said he believed that her brother, Premawardan, had been misled by his grief and sadness over losing a sister.

"He thinks her face is similar," he said. "Maybe he has become too obsessed."

After several days during which the woman remained silent and anxious, Mr Wawlanbokke took her to a local clinic which then transferred her to the Unawatuna Mental Health Hospital near Galle, a colonial-era town south of Colombo.

Medical staff there, who addressed the woman as "Padma", indicated that the woman was being given diazepam, commonly used to treat anxiety. They said that DNA tests were being carried out by a regional hospital.

"When she came here she could not speak, but she started speaking this morning," said one member of the medical team, who asked not to be identified. "Some of the words are difficult to understand."

"Padma" was sitting on the edge of her bed in the women's ward eating biscuits and drinking water from a pink plastic mug. Through the open windows one could see luxuriant banana trees and hear the sound of birds.

She was thin, slightly built, had grey hair and appeared to be aged in her 40s or 50s. Staff said she had been carrying no identification papers or anything else to suggest who she was. She appeared confused and said she was tired.

She was shown a series of photographs, including several from her wedding day. Without being prompted she identified a photograph of her elder sister, Karunawathiee, as her aunt, and when she was asked what her own name was, she replied "Karunawathiee".

She claimed to have two children, though she said they were girls rather than boys.

As well as flashing a smile when she was shown a picture of Padma and her husband, the woman grinned when shown of a photograph of Padma and a group of fellow nurses. "I was a nurse," she said.

Whatever the outcome of the tests, the mystery over the identity of the woman will re-open old wounds for the people of Sri Lanka five years after the most unimaginable natural disaster struck its shores.

The population is already traumatised by a long and bitter civil war that was only recently ended, and those who have studied the aftermath of the tsunami say that mental health counselling was one area that was often overlooked.

And if the confused woman sitting in the hospital is not Padma, then she is a relative of someone else, who may have lost contact with her, whether as a result of the tsunami or for a more mundane reason.

As she sat with a photograph of Padma on her lap, 67-year-old Mrs Wawlanbokke, said of the woman who may turn out to her younger sister: "Everybody is hoping she can get better."