He emerged from the Metro at the L'Enfant Plaza station and positioned himself against a wall beside a trash basket. By most measures, he was nondescript: a youngish white man in jeans, a long-sleeved T-shirt and a Washington Nationals baseball cap. From a small case, he removed a violin. Placing the open case at his feet, he shrewdly threw in a few dollars and pocket change as seed money, swiveled it to face pedestrian traffic, and began to play.