The Transport Ministry said 62 people had been on board the two coaches. The train was travelling from the capital Sofia to the northern town of Kardam.
"Some people could not leave the burning carriages. There are at least three deaths. We suspect the number of casualties may increase," Oleg Petkov, head of the state railway company BDZ, told the national broadcaster BTv.
The cause of the fire was not immediately known.
Nine passengers were injured, four of which were admitted to hospital with burns, officials said.
"Everything happened very quickly and the fire spread rapidly. We were running, leaving the train barefoot, some people were climbing out of the windows in their underwear," said Marina Novacheva, who was in one of the carriages.
"There were only three fire-extinguishers on the whole train and only one of them was working. This was the biggest problem," she told TV channel Nova Televizia.
A spokesman for BDZ said railway workers used five fire-extinguishers but failed to put out the fire as it spread very rapidly.
Güncelleme Tarihi: 29 Şubat 2008, 12:18