Reader Soccer Mom In Denial asked me who these guys were... so I thought I would share.
The Silent Drill platoon is an infantry platoon that is part of the battalion that makes up the bulk of the Marines at Marine Barracks, Washington, DC. For the most part, all of the troops are ceremonial. There are two companies of infantry that comprise the "marching battalion", as well as The Drum & Bugle Corps, The Marine Band, Headquarters & Service Company, etc...
During the spring and summer, the battalion puts on two parades per week, in addition to any other ceremonies (eg, White House stuff or funerals at Arlington National Cemetery). These parades are the Sunset parade, held on Tuesday evenings at the Marine Memorial in Arlington and the Evening Parade, held Friday evenings on the quadrangle of the main barracks area.
The video below, is from a performance during a summertime Evening Parade.
Oh, the area where the Marines are marching is only about 20 yards from the sign you saw on the earlier post, that says "Marine Barracks".