It was snowing on the 14 th Street Bridge and traffic had ground to a standstill as thousands of federal workers and other rush-hour commuters tried to get home ahead of a major storm. With an awful metallic crack, a blue-and-white jet swept out of the swirling snow at 4 p.m., smacked against one of the bridge's spans, sheared through five cars like a machete, ripped through 50 feet of guard rail and plunged nose first into the frozen Potomac River. Moments later in a crowded subway car underneath the National Mall: The train reversed direction.... with a loud popping and crunching sound and a sudden showering of sparks and electrical arcing.... Dozens of people of both sexes screamed. Slowly, surrealistically, the concrete abutment grew larger, closer and actually pressed the left center-rear of the car. The side and roof slowly caved in, almost as a foot crushes a tin can. More screaming, arcing, then silence.