The driver of a train that collided with a bus in Thailand's capital, killing eight people, has tested positive for drugs and was charged with reckless driving, police said on Monday.

The freight train crashed into the public bus on Saturday afternoon at a busy junction in Bangkok, with the vehicle bursting into flames and leaving another 30 people injured, authorities said.

Police charged the train driver, who was also injured, and a railway-crossing guard on Sunday with negligence causing injury and death, said Urumporn Koondejsumrit, chief of the local police station where the crash occurred.

Both denied the charge.

Urumporn said an initial urine test of the driver found illicit drugs in his system, without specifying which substances.

Further testing would include the other driver and a technician aboard the train.

Images on social media on Saturday showed the train approaching a level crossing at a moderate speed before colliding with the bus, which was on the tracks and stuck in traffic.

Bangkok police chief Siam Boonsom said on Sunday there was traffic congestion at the crossing daily but never an accident.

Police were reviewing footage of the scene to determine whether the crossing guard was negligent, he said.

"We see the official holding the red flag which means the track was not safe but we also see that the train did not stop or slow down, causing the crash," Siam said.

Footage of the crash site on other days showed vehicles stopped on the track, with officials raising the red flag to signal to oncoming trains to stop until the traffic cleared, he said.

Authorities were also investigating the speed of the train and its braking distance, Siam added.

Local emergency services said eight people were killed and 30 injured, with 17 still in hospitals as of Sunday evening. (AFP)


Edited by Tony Sabine