At least 275 people have been killed and nearly 1,200 more injured in a horrific three-train pile-up in the eastern Indian state of Odisha.
The high-speed Coromandel Express hit a stationary freight train, jumped the tracks and derailed the Yesvantpur-Howrah Express passenger train passing in the opposite direction just before 7pm on Friday near Bahanaga Bazar station.
It was among the deadliest train incidents in India's history, and the worst in decades.
Here's what we know so far.
How did it happen?
Jaya Varma Sinha, a senior railway official, said the preliminary investigations revealed that a signal was initially given to the Coromandel Express travelling from Kolkata to Chennai to run on the main track line.
But for some unknown reason the signal changed and the train — travelling at more than 100 kilometres per hour — diverted onto an adjacent loop and rammed into the freight train, which was loaded with iron ore.
The engine and several carriages of the express train jumped the tracks, toppled and hit the last two coaches of the Yesvantpur-Howrah train, also travelling at 100kph and heading in the opposite direction on the second main track, Ms Varma told reporters.
'God saved me'
At one of the hospitals nearby, Inder Mahato said he could not remember the exact sequence of events, but he heard a loud bang while in the bathroom before briefly losing consciousness.
Moments later when he opened his eyes, he saw through the door that was forced open people writhing in pain, many of them already dead.
Others were frantically trying to get out from the twisted wreckage of his rail car.
For hours, Mr Mahato, 37, remained stuck in the train's bathroom, before rescuers scaled up the wreckage and pulled him out.
"God saved me," he said, lying on the hospital bed while recuperating from a hairline fracture in his sternum.
"I am very lucky I am alive."
Four of Mr Mahato's friends died in the crash, he said.
Shankar Das survived the accident but suffered multiple broken bones.
"I was travelling to Chennai," Mr Das said. "Suddenly I heard a huge sound, then I saw the train derail from the window.
"Everyone rushed toward the door, I was doing the same, then I must have become unconscious.
"The next thing I knew I found myself in a stretcher with a broken leg."
'I am so helpless'
Meanwhile, many desperate relatives were struggling to identify the bodies of their loved ones because of the gruesomeness of the injuries.
Others were searching hospitals to check whether their relatives were alive.
In the same hospital where Mr Mahato was recovering from his injuries, Bulti Khatun roamed outside the premises in a dazed state, holding an identity card of her husband who was onboard the Coromandel Express and travelling to southern Chennai city.
Ms Khatun said she visited the morgue and other hospitals to look for him, but was unable to find him.
"I am so helpless," she said, sobbing.
'Interlocking system' suspected
Indian Railways investigators are probing an electronic track management system that they suspect malfunctioned.
The electronic interlocking system is a safety mechanism designed to prevent conflicting movements between trains.
It also monitors the status of signals that tell drivers how close they are to the next train, how fast they can go and the presence of stationary trains on the track.
The interlocking system should not have allowed the Coromandel Express to take the loop track, said Ms Varma.
She said the passenger trains, carrying 2,296 people, were not going too fast.
Trains that carry goods are often parked on an adjacent loop line so the main line is clear for a passing train.
The drivers of both passenger trains were injured but survived, she added.
"The system is 99.9 per cent error free. But 0.1 per cent chances are always there for an error," Ms Varma said.
To a question whether the crash could be a case of sabotage, she said "nothing is ruled out".
A sprawling network
Indian Railways, the world's fourth largest rail network and the largest under one management, runs some 14,000 trains daily with 8,000 locomotives over a vast system of tracks some 64,000 kilometres long.
Several hundred accidents occur on the network every year.
In 1995, two trains collided near New Delhi, killing 358 people in one of the worst rail accidents in India. In 2016, a passenger train slid off the tracks between the cities of Indore and Patna, killing 146 people.
Experts say that while accidents have reduced over time, needed improvements have been incremental at best
Derailment accounted for 69 per cent of accidents, according to a 2022 report by the country's top audit institution, the Comptroller and Auditor General of India.
Many of them were caused by defective tracks, poor maintenance and old signalling kit combined with human errors, the report said.
Prime Minister Narendra Modi has pushed a $US30 billion ($45 billion) railway infrastructure modernisation in a bid to boost India's economy and connectivity, with trains the preferred and cheapest mode of long-distance travel for both people and goods.
On Saturday, Mr Modi viewed the mangled wreckage and offered condolences to the families of those killed and visited some of the hundreds of injured.
He had been due to launch a modern electric train fitted with a state-of-the-art safety feature to prevent collisions.
What happens next?
Railway Minister Ashwini Vaishnaw said on Sunday that India's Railway Board had recommended the investigation be handed over to the country's federal investigating agency.
While visiting the crash site, Mr Modi vowed to prosecute anyone found to be responsible for the crash.
"This incident is very serious," he said.
"Orders for a thorough investigation have been issued and whoever is found guilty will get the strictest punishment. That person will not be spared."
Experts said the crash showed India's complex and often antiquated railway system still had far to go.
"Pure operational failures are not uncommon on Indian railways," former top railways official Subodh Jain told AFP.
"Safety mechanisms are now far more robust, but it's a work in progress."
Mr Jain said the pressure of traffic is so great it is often hard to "block a line long enough" to carry out necessary improvements.
"There has to be some drastic change," he said.
ABC/wires
