Hundreds of people were forced to evacuate the L’Enfant Plaza station during Monday’s afternoon rush hour.
Metro Transport Police said that there would be an investigation into the cause of the smoke.
The Washington Metro system serves a population of five million, including transport services to neighbouring states.
The majority of people admitted to hospital were suffering from smoke inhalation, according to Metro and hospital officials.
Passenger Jonathan Rogers told The Washington Post that it “started to get scary pretty quick”.
“People started praying. Smoke was coming in pretty steadily,” he added.
The incident forced a closure of many of the Metro lines, but some lines have since reopened.
Washington DC’s Metrorail is the US’s second busiest mass transit system after New York.
The National Transportation Safety Board are expected to hold a news conference on Tuesday.