The James Street swing bridge in Thunder Bay remains closed as an investigator from the Ontario Fire Marshal's office surveys the scene.
CN Police have cut off access to the bridge while the investigation continues. The bridge is considered a crime scene until the cause of the fire is determined.
John Montgomery, a representative of the Fire Marshal's office, and a Thunder Bay fire inspector were examining the damage and taking photos on Thursday.
Montgomery wouldn't speculate on what caused the fire, but he said the damage was significant.
"On the north side of the bridge, down the shore of the river, that's supported by ... a set of wooden trestles," he said, "and those have been damaged somewhat."
Montgomery said he collected most of the evidence he needed, but wouldn't estimate how long it would take for a forensic analysis to be completed.
Workers with CN are now assessing the damage to railway property.
In the meantime the bridge remains closed and travellers must use Chippewa Road to enter or leave the Fort William First Nation.