CN Rail dealt with what it calls a "non-accident release" Monday night.
Sarnia Fire and Rescue Service responded to the rail yard after an odour was detected from a rail car around 8pm.
It was determined the smell was coming from the top hatch of a car containing ethyl benzene. The hatch was sealed and no further action was needed.
CN says there was no environmental impact.
No formal public notification of the incident was given.