(Bloomberg) -- India’s deadliest rail accident in nearly three decades has renewed focus on the safety issues that plague one of the world’s biggest railway networks.

The three-train collision that left at least 275 people dead and more than 800 injured comes as Prime Minister Narendra Modi has been promoting a push to improve India’s infrastructure. His government has touted new, super-fast trains and plans to make the railways carbon neutral by 2030.

Here’s everything we know about the accident so far:

How did the crash happen?

An express passenger train collided with a stationary goods train in the Balasore district of the eastern state of Odisha on June 2. This resulted in several carriages being thrown on to an adjoining track and into the direct path of another express passenger train coming from the opposite direction.

The impact of the first collision was so powerful that carriages of the Coromandel Express were thrown high into the air, twisting and then smashing off the tracks. Photos and videos from the site showed mangled metal strewn across the tracks.

Rescue workers had to use gas torches and electric cutters to reach the trapped survivors. 

How many people were killed and injured?

The latest government death toll is 275, revised down from an earlier number of 288. At least 800 people were injured, many of them grievously. 

The search and rescue operation ended Saturday and more than a 1,000 people started working to clear the tracks. Using hydraulic excavation machines and earth movers to clear the wreckage railway operations at the accident site resumed late Sunday.

What caused the crash?

Investigators are looking into causes behind the accident including, human error, signal failure, or sabotage.

“It seems there was some error in the signaling system,” Jaya Varma Sinha, a senior railway official, said at a press conference in New Delhi on Sunday. 

The Coromandel Express was meant to enter the Balasore station on the main line but received a signal to use a loop line where a freight train was parked, the Press Trust of India news agency reported, citing a preliminary investigation report.

Railways Minister Ashwini Vaishnaw has hinted that sabotage could also be involved and asked that the accident be probed by the Central Bureau of Investigation.

How safe are India’s trains?

The Indian Railways, the fourth largest network in the world, carries about 24 million passengers daily and is crucial to the the country’s economy.

However, the railway system put in place during British colonial rule has seen limited upgrades and modernization and is plagued by hundreds of accidents each year. Most are blamed on outdated signaling equipment or human error. 

The Comptroller and Auditor General of India in a recent report has flagged multiple shortcomings, including inadequate inspections and a lack of timely maintenance.

Last year, the railways introduced a new collision prevention system known as Kavach, the Hindi word for armor. It controls the speed of a train by automatically applying the brakes in case of over speeding or a collision-like situation. However it’s installed on a limited number of trains and routes. 

The anti-collision system was not in place on the route where Friday’s accident occurred.

About 3,000 people have been killed in train accidents since 2000. In 1995, more than 300 people died in a collision. In 1981 about 750 passengers were killed when a packed passenger train got derailed on a bridge and plunged into a river, in what’s believed to be India’s worst-ever rail disaster. 

How has Narendra Modi reacted to the crash?

Modi, who was supposed to inaugurate a high-speed train service in western India on Saturday, visited the accident site and a nearby hospital. He reviewed rescue and relief efforts.

The opposition parties, especially the Congress party, have blamed the government for not doing enough to improve rail safety over the last decade. They demanded the Railways Minister Vaishnaw resign. 

Modi faces no immediate political fallout from the tragedy. His ruling Bharatiya Janata Party managed to to hold on to power in his home state of Gujarat last year despite a deadly bridge collapse that left at least 135 people dead.

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