The term 'Erie Canal Mule of Song' refers to the character of Sal, a mule featured in the folk song 'Low Bridge, Everybody Down'. The song is about the Erie Canal, a 363-mile waterway connecting the Hudson River to Lake Erie. The song tells the story of a mule driver named Sal, who is taking a load of cargo down the canal. The song is a popular folk tune and has been covered by many artists over the years.