The federal government shut down the Sky Express bus company indefinitely following a crash on Tuesday which killed four passengers and injured 50. The bus, traveling from Raleigh, North Carolina to New York City, was one of three major crashes involving discount, long-distance bus companies in the Northeast in May alone.   These bus accidents are a growing threat, not only because discount bus lines have been exploding in popularity over the past five years, because they are subject to very little federal safety oversight. WNYC's Transportation Nation reporter Jim O'Grady says that prior to its closure, the Sky Express Bus Company had the worst record in the country for driver fatigue and falsifying driving records. He explains that the company was about to be shut down, when the crash happened.