John Carroll provided video of the incident to FOX 5. He says it happened on the Red Line at around 11:20 Sunday morning between Metro Center and Farragut North.
Metro says a customer reported the issue to the operator, who took the train out of service at Dupont Circle. All passengers boarded the next available train. No one was hurt.
The train has been taken to a maintenance facility for diagnostic testing, according to Metro's Dan Stessel.
Metro says it was a 4000-series car, which is the transit agency's least reliable fleet. Stessel said all 4000-series railcars will be replaced by 7000-series cars in the coming years.