A man who apparently was asleep with his right leg on a railroad track when a passing train severed it managed to crawl nearly 300 yards in search of help and was found alive several hours later, police said. Guillermo Tascano, 22, was listed in serious but stable condition at County-USC Medical Center, a nursing supervisor said. A transient who saw Tascano lying on the ground near 2555 Olympic Blvd. notified a nearby Sears department store security guard, who called police and paramedics. The nature of his injury was such that the main arteries and vessels in his leg contracted. That, coupled with shock, kept him alive, Los Angeles Fire Department spokesman Greg Acevedo said. Doctors tried unsuccessfully to reattach the leg.