An odour coming from a rail car in the Sarnia rail yard prompted a precautionary community alert about a possible isobutane vapour leak just after 6am Thursday.
A CN spokesman says no liquid leaked and the smell in the area of Russell St. S. and Campbell St. was contained after a valve was tightened.
Sarnia Fire and Rescue Service also responded as a precaution and authorities responsible for dangerous goods were notified.
The all clear was issued by 7:30am.