DC Metro User's Guide: Entering a DC Metro subway station
Station gates require a Farecard or SmarTrip card to pass through. Lights on each gate indicate whether the entrance is operating as an entry or exit, with a green light in the direction of travel.
With Farecard or SmarTrip card in hand, it is time to enter the DC Metro station and find your platform. Gates, like those pictured here, will accept your pass and let you enter or exit. Green or red lights indicate the current direction of each gate. Use gates marked green to pass through in the direction you are headed.
Farecards should be inserted into the front of the gate, and will emerge on the top. The gate opens when the card is removed. SmarTrip cards can simply be waved over the white detector on the top or side of the gate. Note that you may only use one card per person, and need to use the same card when you exit Metro as you used to enter. At some stations, gates, like the four right most gates shown in this picture, are designed to only accept SmarTrip cards.