At least 120 people died, many in their sleep, and more than 200 were injured after a passenger train derailed early yesterday in the northern state of Uttar Pradesh.
At least 14 carriages of the Indore-Patna Express train jumped off the track at around 3am in Pukhrayan, 60km from Kanpur.
Television images showed the coaches piled up against each other, with at least three of them crushed.
Through the day rescue workers reached survivors trapped in the three severely damaged train cars, where up to 150 people may have been at the time of the accident.
“We have to go slow, some people may still be alive and we are doing cold cutting through the metal to reach them,” Indian Railways spokesman Anil Saxena said.
By evening, as the light faded, Saxena said the rescue operations had almost concluded.
At least 75 people were critically injured, another 150 had minor injuries, senior police official Zaki Ahmed said.
The train with 23 coaches was carrying around 2,500 passengers who were to disembark at various stations and mostly at the terminal station of Patna in Bihar, where distressed relatives had gathered to await news of their loved ones.
“We suddenly woke when the train stopped with a massive jolt and screeching noise, we managed to get out and many of the bogies had fallen off the tracks,” one man told NDTV network.
Television channels showed streams of people carrying their luggage across fields to a highway where buses were waiting to take them to the nearest town.
Stranded passengers were being taken to the next station at Malasa where a train was being arranged to take them to their destination,
Railway Minister Suresh Prabhu said.
It was not clear at this stage what caused the accident, Saxena said, adding that the entire focus was on rescue and relief.
“I woke up suddenly around 3.10am and felt a tremor. The train came to a screeching halt,” survivor Yaqoob Ahmed told the Hindustan Times newspaper from a hospital in the nearby city of Kanpur.
“All of a sudden, I was crushed under a crowd of people... everyone was screaming for help.” 
Survivors also told of their desperate search for loved ones on the train, which was carrying at least one wedding party with the marriage season in India in full swing.
Hundreds of army and police have been deployed at the scene, where rescue workers used gas-powered metal cutters to slice through severely mangled coaches to try to get to survivors.
“We have been able to pull out 24 people, out of which five were found to be alive,” said Brigadier A Chhibbar, who is leading the army’s rescue operations.
“We will carry on day and night, till there is any inkling of even a single person being pulled alive.”
It is the worst disaster since 2010 when a passenger train crashed into a freight train in West Bengal, killing 146 and injuring over 200.
Authorities have launched an official investigation into the accident, which junior railways minister Manoj Sinha said may have been caused by damage to the tracks.
India’s railway network, one of the world’s largest, is still the main form of long-distance travel in the vast country, but it is poorly funded and deadly accidents occur relatively frequently.
A 2012 government report said almost 15,000 people were killed every year on India’s railways, describing the deaths as an annual “massacre”.
Nitika Trivedi, a student who boarded the train with her family, said images of the victims’ bodies would long haunt her.
“I had never seen anything like this in my life before. I am shaken to the core,” she said.
Anxious relatives thronged the station in Indore in Madhya Pradesh where the train originated, many clutching pictures of their loved ones, and railway officials said special trains had been deployed for stranded travellers.
“We are also trying to clear the tracks and complete the restoration work as quickly as possible,” Vijay Kumar, a spokesman for north-central railways, said.
Local media reports said the train was packed with families, some of them travelling home for weddings.
Bride-to-be Ruby Gupta, who survived the accident with a fractured arm, was desperately searching for her father.
“I have been looking everywhere for him,” she told the Press Trust of India.
Another survivor said the train stopped a couple of moments after 3am for unexplained reasons.
“It then suddenly picked up speed,” the man said. “And then I got an eerie feeling that the train was rolling down a valley,” he said.
“By the time I learnt what had happened, some 20-25 people in my coach had been killed. A six-year-old girl was literally cut into two pieces.”
In 2014, an express train ploughed into a stationary freight train, also in Uttar Pradesh, killing 26 people.
And last year, 27 people died when two trains derailed in Madhya Pradesh during heavy rain.
Prime Minister Narendra Modi’s government has pledged to invest $137bn over five years to modernise the crumbling railways, making them safer, faster and more efficient.
By some analyst estimates, the railways need Rs20tn ($293.34bn) of investment by 2020, and India is turning to partnerships with private companies and seeking loans from other countries to modernise its network.
Last year, Japan agreed to provide $12bn of soft loans to build India’s first bullet train.
On Sunday Modi took to Twitter to express his condolences.
“Anguished beyond words on the loss of lives due to the derailing of the Patna-Indore express. My thoughts are with the bereaved families,” Modi said.