"Slaves sung the spiritual, The Gospel train’s a’comin’, to alert other slaves that a group was preparing to escape and travel north to freedom. “Gospel Train” was code for the Underground Railroad.
Plantation owners would be unaware their slaves were planning to escape; slave songs were part of the day’s routine. A plantation owner would simply hear the religious and Biblical references and assume the slaves were singing for spiritual reasons." - http://www.osblackhistory.com/gospeltrain.php