A freight train plowed into heavy traffic that was crossing train tracks in Bangkok on Saturday, killing at least eight people and injuring dozens of others, according to Thai officials.

The train hit a bus just before 4 p.m. local time, setting off a crash with cars and motorcycles and sparking a fire that engulfed some of the vehicles. All eight of those who died were on the bus, according to the authorities, who said that 32 other people were injured.

It was not immediately clear what caused the crash, which took place on one of the busiest roads in the Thai capital, near a train station linking the city to an international airport. The traffic, which is typically heavy even on weekends, crosses the train tracks.

The train conductor and the bus driver were in police custody pending an investigation, said Police General Kitrat Phanphet of the Royal Thai Police. Emergency personnel at the scene had previously said that the bus driver had died in the crash.

“I heard the alarm on the incoming train, but usually the train that passes this road waits until the traffic is cleared,” Artit, a survivor of the crash who fled his car when the fire broke out, told local media, which gave only his first name. “It took about 10 minutes for my car to completely burn down.”

The train’s driver was injured and was in a hospital, rescuers on the scene told local media. Transport officials said that they would conduct an investigation into the cause and recover the train’s data recorder to see how fast it was moving.

Though they said it was too early to draw conclusions, they said that a barrier meant to separate the train from the traffic had not descended.

“Usually the barrier is the first thing to come down before other signals are given,” said Siripong Angkasakulkiat, the Thai deputy minister of transport, adding that the authorities were investigating. “Someone has to answer for this, but we will wait for scientific evidence.”

Anan Phonimdang, an official for Thailand’s state-owned rail operator, said in a news conference on Saturday that a barrier, which is typically operated by an officer, could not be brought down because there were vehicles parked on the tracks. “Usually, you’re not allowed to park within five meters near the track, and the train couldn’t stop in time,” he said.

Surachit Intha, a motorcycle taxi driver, said he had been parked near the train tracks when the crash happened. “I heard the train and wondered why the barrier didn’t come down, and the next thing I saw was the train was dragging those vehicles,” he said.

Mr. Intha said that he and others rushed to help the injured and get people out of their vehicles and that he was saddened by the news that people had died. “This incident shouldn’t have happened,” he said.

On Saturday evening, at least 100 police officers and other emergency workers remained at the intersection. Much of the scene was blocked off, with green material keeping the burned bus out of sight. Emergency workers could be seen removing a body wrapped in a white sheet. A part of a train remained on the rails, and a gray car sat on the road, its sides and front crushed.

The bodies of the victims had been badly burned, said Piyarat Tinkaew, an official with the Ruamkatanyu Foundation, a volunteer rescue group in Bangkok that was responding to the crash. “We will do our best to bring those bodies back to their families.”