Train derails in flood waters
In pictures
Rescue workers have been searching for people trapped in carriages amid fears the death toll could rise further.
Initial reports suggest an irrigation tank burst, causing a bridge to collapse, derailing seven carriages.
The crash occurred early on Saturday south of Hyderabad, the state capital of Andhra Pradesh.
Heavy rains have killed more than 100 people in south India this week.
Holidaymakers
The Delta Express crashed near the town of Veligonda in Nalgonda district, about 30km (18 miles) south of Hyderabad.
Seven of the 14 coaches and the engine of the train were derailed and at least three coaches fell into the flood waters.
Divers from the air force and navy used blow torches to cut through the wreckage to free the scores of people still feared trapped. Some survivors were clinging to luggage racks and ceiling fans.
We were fast asleep, when there was a big bang and a thud. The next thing the train was under water
P Ramesh, survivor
Hundreds of passengers have been rescued so far but television pictures also showed bodies of men, women and children laid out on the banks.
After surveying the site of the crash, India's junior railway minister, R Velu, told a news conference in Hyderabad that "little could have been done to avoid the accident".
The BBC's Omer Farooq, who is at the scene, says many of the victims were holidaymakers travelling to celebrate Diwali, the festival of lights, which takes place on Tuesday.
Srinivas Rao and his cousin, Gangadhar, were travelling on the train with their parents and managed to escape.
"But we don't know what's happened to our parents," they told the BBC.
P Ramesh, a passenger who lost seven members of his family, including his wife and brother, told Associated Press: "We were fast asleep when there was a big bang and a thud. The next thing the train was under water.
"It was pitch dark and people were screaming."
Heavy rains
Rains have lashed southern India for more than a week, claiming more than 100 lives in Andhra Pradesh, Tamil Nadu and Karnataka.
India's state-run railway system carries more than 13m passengers a day.
It has one of the world's largest rail networks, but also a poor safety record.
About 300 rail accidents are reported every year, resulting in a high number of casualties.
Earlier this month, at least 16 people died and dozens were injured in a train crash in Madhya Pradesh state.
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The BBC and CNN has reported that a train derailment Saturday morning at about 4:15 a.m. local time (Friday 2245 UTC) has resulted in at least 100 casualties in the state of Andhra Pradesh in Southern India. Heavy rains, which killed over one hundred people in India this week, apparently washed out a section of track. A rescue operation is ongoing as some train carriages overturned into the flood waters and some people are apparently trapped. The BBC reports that many of the passengers were traveling to celebrate Diwali, the festival of lights, which occurs Tuesday.

India operated rail systems carry more than 13 million passengers a day, but has a poor history of safety. The BBC says that about 300 rail accidents are reported each year, resulting in significant casualties.