«Underground Railroad». dictionary.com. [Consulta: 17 juliol 2011]. «A network of houses and other places abolitionists used to help slaves escape to freedom in the northern states or in Canada...—American Heritage Dictionary»
safepassageohio.org
«Underground Railroad Codes». Myths and Codes of the Underground Railroad p. 20. Greater Cincinnati Television Educational Foundation. [Consulta: 29 juny 2013].
American Civil War Census Data This source derived numbers for "% of Families Owning Slaves" by assuming only one slaveholder per family, a presumption with evidence only to the contrary. The summary numbers also include 17 slaves and 8 slaveowners counted by census-takers in Nebraska and Kansas, where slavery was illegal.