The majority of the train cars involved in the June 28 derailment in the St. Clair Tunnel have been removed.
On Friday, a week after the derailment, nine of the 46 cars were still in the tunnel between Port Huron, Michigan and Sarnia.
CN Rail said a team of dangerous goods experts and contractors were still cleaning up and pumping out spilled sulphuric acid before the remaining cars can be taken out. A timeline for that was still unknown.
No injuries were reported in the derailment and the spill was contained to the site.
The cause of the derailment is still under investigation, which according to officials could take over a year to complete.
The Transportation Safety Board of Canada (TSB) announced Friday that it will be conducting the investigation.
"Following an initial assessment of the accident by both the TSB and the US National Transportation Safety Board, the derailment was initiated in Canada," the TSB said in a news release.
While the tunnel is out of use, CN Rail has an additional six freight trains per day being rerouted through Windsor.