The driver of a freight train involved in a fatal collision with a public bus in Bangkok has tested positive for drugs and been charged with reckless driving, authorities say.

The collision, which happened at the Asoke-Din Daeng railway crossing on Saturday afternoon, saw the train crash into a public bus that had come to a stop on the railway tracks.

The bus burst into flames, killing eight people and injuring dozens. Initial reports suggest the bus had become stuck on the tracks due to heavy traffic, which prevented the crossing barriers from lowering properly.

The Asoke-Din Daeng railway crossing is notoriously hectic, and leads to one of the busiest intersections in Bangkok.

Both the bus driver and the guard responsible for lowering the barrier - which is done manually - have also been charged, police said.

Authorities have ordered mandatory drug and alcohol tests for all train drivers and railway staff before their shifts, said Pichet Kunadhamraks, director-general of Thailand's rail transport department.

Police did not specify what drugs had been found in the urine test of the train driver.

Pichet said preliminary findings from the train's black box showed its emergency braking system was only activated about 100m (330ft) away from the bus, which was not enough to avoid collision, Thai PBS reported.

Dr Amorn Phimarnmas, president of the Structural Engineers Association of Thailand, tells the BBC he estimates more than 100,000 road vehicles pass through the Asoke-Din Daeng railway crossing every day - above the safety threshold for such crossings.

At the crossing, motorcyclists are often known to weave through safety barriers to beat traffic and avoid getting slowed down by passing trains.

The tracks were built long before the surrounding roads, and the city has grown around them, giving way to "risk normalisation", Amorn says.

"It's when risk becomes routine. People think 'it'll be fine' and carry on, until one day we end up with exactly the kind of disaster we've just seen."