In “Cash on the Barrelhead”, a Louvin Brothers song  he recorded for his album Grievous Angel, Gram Parsons (the man who invented country rock) sings of a bus driver reminding him that “this ol’ gray dawg gets paid to run”. Greyhound, though, is still one of the cheapest forms of long-distance transport in the US, which is why it’s the choice of what Doug Levitt calls “people on the margins”. Levitt, who describes himself as a former foreign correspondent and “downwardly-mobile” singer songwriter, has clocked up 120,000 miles riding Greyhounds across the United States over the past 12 years, on an odyssey inspired by Woody Guthrie. In The Greyhound Diaries, for the BBC World Service, he trades stories with passengers he meets along the way, and turns some of theirs into songs.