Metro told passengers to expect fewer trains, longer waits and crowding on trains Thursday.
Officials warned riders that delays may occur because of a possible problems with rail car inspections, according to a statement.
Rail cars are inspected every day, Metro said.
Metro officials say ATU Local 689, a union for Metro employees, was concerned about the inspection procedure for the new 7000-series rail cars. Metro says it voluntairily stopped inspections until the safety concern is resolved.
The Washington Post reports that a mechanic was shocked during an inspection, and now union members are refusing to inspect rail cars.
An update is expected this afternoon, Metro said.
