Metrorail service was suspended Saturday night on part of the Red Line after a haze of possible smoke was reported in the train tunnel near the Bethesda station, authorities said.
No fire was found, according to the Montgomery County fire and rescue service.
Metro issued an alert saying that service had been suspended between the Medical Center station in Bethesda and the Van Ness station on Connecticut Avenue in the District.
Metro cited a “track problem” but gave no details. It was not immediately clear whether the problem was connected to the haze reported at Bethesda.
Montgomery fire and rescue spokesman Pete Piringer said in a tweet that an emergency response team from Metro was entering the tunnel in the Bethesda area.
He said it appeared likely that a problem with a stud bolt was involved. The smoldering of such bolts has disrupted service in the past. One possible cause was fraying insulation and leaks of electrical currents. The bolts fasten rails to the trackbed.
Metro began to set up shuttle bus service between affected stations.