Investigators zeroed in on whether a blaze on the train a few hours before the disaster set off the deadly chain of events.
The death toll rose with the discovery of two more bodies Tuesday. About three dozen more people were missing.
The bodies that have been recovered were burned so badly they have yet to be identified.
The Montreal, Maine & Atlantic Railway train broke loose early Saturday and hurtled downhill through the darkness nearly seven miles before jumping the tracks at 63 mph in Lac-Megantic, near the Maine border, investigators said. All but one of the 73 cars were carrying oil. At least five exploded.
The blasts destroyed about 30 buildings, including the Musi-Cafe, a popular bar that was filled at the time, and forced about a third of the town's 6,000 residents from their homes.
Rail dispatchers had no chance to warn anyone during the runaway train's 18-minute journey because they didn't know it was happening themselves, Transportation Safety Board officials said Tuesday. Such warning systems are in place on busier lines but not on secondary lines, said TSB manager Ed Belkaloul.
Resident Gilles Fluet saw the approaching train just before the derailment and explosions.
"It was moving at a hellish speed," he said. "No lights, no signals, nothing at all. There was no warning. It was a black blob that came out of nowhere."
The same train caught fire hours earlier in a nearby town, and the engine was shut down standard operating procedure dictated by the train's owners, Nantes Fire Chief Patrick Lambert said.
Edward Burkhardt, president and CEO of the railway's parent company, Rail World Inc., suggested that shutting off the locomotive to put out the fire might have disabled the brakes.
Efforts continued Tuesday to stop waves of crude oil spilled in the disaster from reaching the St. Lawrence River, the backbone of the province's water supply. Environment Minister Yves-Francois Blanchet said the chances were "very slim."
Also Tuesday, about 1,200 residents were being allowed to return to their homes.