In total, 31 victims were taken to hospital, 11 in critical condition.
Another three arrived at hospitals on their own. None of the 103 passengers on the train was seriously injured.
Eleven Transportation Safety Board investigators were at the scene, being assisted by Ottawa police.
Their investigation includes retrieving the event recorders known as black boxes from the train and the bus, and the device that records what happened with the guard arms at the crossing.
Eyewitnesses reported the bus did not seem to slow down.
Some on board said passengers were screaming at the driver to stop while another said he saw the driver desperately slam on the brakes before the impact.