PAGEDALE, Missouri — After two men brutally assaulted a security guard on a desolate train platform on the outskirts of St. Louis, county transit police detective Matthew Shute struggled to identify the culprits. He studied grainy surveillance videos, canvassed homeless shelters and repeatedly called the victim of the attack, who said he remembered almost nothing because of a brain injury from the beating.Months later, they tried one more option.Shute uploaded a still image from the blurry video...