The Louisville and Nashville Railroad extended its railroad into Harlan County in 1910, marking the beginning of the rapid development of one of the leading coal fields in the United States. Harlan County quickly emerged from an obscure, backwoods mountain county into an industrial field recognized across the country for its superior grade of coal. Low in ash and sulphur, Harlan coal was regarded at the time as the best all-around coal in the world and quickly contributed to building the nation.