At least 31 people died and 25 were injured when two trains derailed within minutes of each other while crossing a small bridge in Madhya Pradesh shortly before midnight on Tuesday.
The Kamayani Express, headed for Varanasi in Uttar Pradesh from Mumbai, derailed near Harda town, 160 km south of Madhya Pradesh capital Bhopal, shortly before midnight.
The Janata Express, en route to Mumbai from Patna, too derailed at the same spot minutes later.
Officials engaged in the rescue operations said they feared the death toll could rise. Survivors said some persons could have been washed away by the swollen waters of the Machak river.
The accident occurred at Kudwa, about 18 km from the district headquarters Harda, located between Bhirangi and Khirakia railway stations.
Harda district public relations officer Vitthal Maheshwari said 31 people were killed, including 10 women and five children.
The chairman of the Railway Board, AK Mittal, said on Wednesday that the derailment of the two trains near Harda in Madhya Pradesh may have been caused by a sudden flash flood.
Mittal said that ten minutes before the unfortunate tragedy, spot train movement was absolutely normal, and there was no inkling about the accident taking place.
An inquiry was ordered into the incident and the commissioner of railway safety (central zone) will conduct the probe.
Watch:Railway Chairman says sudden flash floods led to MP train tragedy
The railways announced a compensation of Rs 2 lakh for the next of the kin of the dead and Rs 50,000 to those who sustained serious injuries.
As rescuers initially grappled with darkness and inclement weather, survivors recounted the midnight horror .
Manoj Mongi, a resident of Muzaffarnagar who was on board Kamayani Express, said: "I was sleeping and suddenly I felt a jolt...I came out. I saw three women floating, but could not save them."
Tax consultant Shashi Bhushan Pandit, another passenger, said: "The water level on the track was almost waist-high. The current was swift...I cannot say how many people are trapped."
Though the exact reason for the derailments is not yet known, railway officials said heavy rains in the past few days might have damaged railway tracks.
"There was water on the tracks and the bridge was submerged. So, the last bogies of Kamayani Express got derailed. Simultaneously, on the other track, the engines and coaches of the Janata Express also got derailed," railway spokesperson Anil Saxena said.
TV news channels reported some bogies of Kamayani Express fell into the Kalimachak river but Madhya Pradesh chief secretary Anthony de Sa did not confirm these reports.
"However, we are not taking chances and have alerted army units in Bhopal and Mhow. We have requested the defence ministry for help from army too. Teams of National Disaster Response Force and the State Disaster Response Force have left for the spot," he told Hindustan Times.
As the news of the accident broke, railways minister Suresh Prabhu tweeted: "Rushing emergency medical & other relief personnel to spot. darkness, water creating hurdles but ordered all possible help. Trying our best (sic).
"Local villagers, Admn r also in rescue operation. Everything that's necessary and possible being done. Constant in touch with CM MP & Rail off (sic)."
Traffic on both up and down lines came to a halt after the accident.
Prime Minister Narendra Modi and Madhya Pradesh chief minister Shivraj Singh Chouhan tweeted about the accident, offering condolences to families of the victims and providing regular updates on the rescue operations.
The two train accidents in Madhya Pradesh are deeply distressing. Deeply pained over the loss of lives. Condolences to families of deceased.— Narendra Modi (@narendramodi) August 5, 2015
We are providing all possible Medical assistance at the site. Medical teams from various cities have been rushed in to assist in R&R ops.— ShivrajSingh Chouhan (@ChouhanShivraj) August 5, 2015
India's railway network, one of the world's largest, is still the main form of long-distance travel in the country, but it is poorly funded and deadly accidents are frequent.
In 2012, a government report said almost 15,000 people were killed every year on railways, describing the deaths as an annual "massacre" due mainly to poor safety standards.
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(With inputs from Mukesh Pandey and Sunil Karhalkar)