The crane was being used to build a bridge for the upcoming high-speed rail line between Bangkok and China via Laos. However, the train that was hit was traveling on existing tracks from the capital, Bangkok, to Ubon Ratchathani on the eastern border with Laos.
Witness Mitr Intrpanya says he heard loud noises, including explosions.
"When I went to look, I saw the crane on top of a passenger train," he told the AFP news agency.
The crane's metal appeared to strike the middle of another wagon, cutting it in two.
According to media reports, the authorities say more than 30 people are dead. More than 60 are reported injured, and some people are missing. According to Transport Minister Phiphat Ratchakitprakarn, 150 people were on board the express train.
The first part of the new high-speed line is due to open in 2028.
