At least five people died and 40 others were reported missing following the Saturday derailment in the Quebec town of Lac-Megantic. A train operated by MMA was carrying 72 carloads of crude oil from North Dakota.
There is not enough pipeline capacity in North America to handle oil production increases, resulting in more crude oil deliveries by rail.
MMA said in a statement Sunday it was unable to determine the exact cause of the accident but was cooperating fully with Canadian investigators.
The company said the train was shut down at the time of the incident because the engineer was away from his station. "[This] may have resulted in the release of air brakes on the locomotive that was holding the train in place," MMA said.
MMA said the blaze associated with the derailment was largely extinguished.
"Provincial and federal authorities have taken control of the derailment area, and MMA personnel have not been able to enter to continue their investigation concerning cause and to plan recovery operations," the company said in a statement.