CHICAGO (CBS) — Red Line trains were diverted from the subway to the elevated tracks downtown for nearly an hour Sunday morning, but have resumed normal service, after a hazardous materials response at the Grand stop.

Trains were halted at the Grand subway stop around 6:30 a.m., and later diverted to the elevated tracks between Fullerton and Cermak-Chinatown, after a thick white film with a strong odor was found on the platform, CTA officials said.

Police and fire department crews responded, and determined the substance was discharge from a fire extinguisher, according to the CTA and the Chicago Fire Department.

Trains resumed normal service in the subway by about 7:20 a.m., although were still bypassing the Grand station until the platform could be cleaned.