Firefighters and rescue crews were dispatched as flames engulfed the public bus and nearby vehicles close to the Airport Rail Link's Makkasan station, officials said, adding that the crash also involved cars and motorcycles.
Preliminary reports showed the bus had been stopped on the tracks at a red light, preventing crossing barriers from closing, Deputy Transport Minister Siripong Angkasakulkiat told reporters.
The train, which was transporting containers, was unable to stop in time to avoid colliding with the bus, he added.
"Eight people were killed and 32 injured, with the wounded being treated at various hospitals. All eight dead were on the bus," he said.
Videos shared on social media showed the train striking the bus and dragging several other nearby vehicles along the tracks.
"The bus was stuck at a red light, so it couldn't move. Cars were also blocked and unable to move forward," Wanthong Kokpho, a motorcycle taxi driver who witnessed the crash, told Reuters.
"The fire broke out immediately ... If this had been a normal working day, the damage would have been much worse."
Rescue teams worked to pull injured victims from the wreckage as fire crews battled the blaze with water hoses, officials said.
The fire was brought under control, and crews were cooling the area, venting gas and continuing to search for victims, they said.
Authorities are investigating the cause of the incident.
Thailand's roads rank among the world's deadliest, according to the World Health Organisation, due to weak enforcement of safety standards.
