There was a fire Tuesday morning near the site of last Friday's derailment at the St. Clair Tunnel.
CN Rail said equipment being used to wreck the scrapped rail cars ignited a fire, about 1.6 kilometres from the site of the derailment.
The company said the fire was contained quickly, no injuries were reported and there was no threat to public safety.
Early Friday morning, as many as 46 CN Rail cars derailed inside the tunnel between Port Huron, Michigan and Sarnia. So far, about 20 cars have been cleared from the site as crews work around the clock to remove the rest.
CN said a team of dangerous goods experts and contractors are still cleaning up and pumping out spilled sulfuric acid.
The rail company said the spill is contained to the site and does not pose any risk to public safety or the environment.
The cause of the derailment is still under investigation.