Mechanics for the nation's third-largest public transportation system went on strike Tuesday, forcing hundreds of thousands of commuters to scramble for alternate transportation for the second time in three years. Metropolitan Transportation Authority mechanics walked off the job after midnight, and bus drivers, train operators and other workers were honoring picket lines. The move - which came on the heels of an unrelated strike at the region's groceries - shut down buses and trains that an estimated 500,000 daily riders count on to get around Los Angeles County. "I'm just stranded," commuter David Strattling, 59, said early Tuesday. "I won't...