India will launch an investigation on Tuesday into a train collision that killed nine people in the state of West Bengal and injured more than 50, a day after a top railway official blamed the incident on driver error.

The death toll was revised down to nine from 15 after Monday’s accident, in which a freight train rammed into a passenger train heading for the state capital of Kolkata from the northeastern state of Tripura.

The investigation by India’s top railway safety official will start on Tuesday, Chetan Kumar Shrivastava, general manager of the Northeast Frontier railway, where the accident happened, told Reuters.

“The inquiry will involve eye-witness accounts, scrutiny of official documents and statements from railway officials, regarding signalling and other mandatory safety issues,” he added.

On Monday, India’s top railway official said the driver of the freight train, who was among the dead, disregarded a signal, leading to the crash with the Kanchanjunga Express, which had halted near a railway station in the district of Darjeeling.

There were 1,400 people aboard, a railway spokesperson said.

But media said an automatic signalling system had not been working from Monday morning, prompting authorities to advise train drivers to proceed slower than usual, in a process known as “paper signals”.

India’s opposition leaders criticised the railway safety record of Prime Minister Narendra Modi’s government, attributing it to negligence.

The incident came a little over a year after about 288 people were killed in one of India’s worst rail crashes in the neighbouring state of Odisha, caused by a signalling error.

State-run Indian Railways, notorious for overcrowding, is the world’s fourth largest train network, carrying 13 million people a day, along with nearly 1.5 billion tonnes of freight in 2022.

In remarks to media on Monday, top railway official Jaya Varma Sinha, who chairs India’s railway board, called for human error to be reduced, adding that an anti-collision system was being set up nationwide.

Partial services resumed on the affected tracks on Tuesday, with some trains diverted and others running slower than usual, railway officials said.