 Mitchell is said to be the only writer who wrote about the Underground Railroad while it was still illegal, although others had described it at an earlier date.The British writer Harriet Martineau had mentioned the concept in 1837 and the British abolitionist Joseph Sturge wrote about it in 1841, but they did not name it.Mitchell's book took the name of the Underground Railroad into his book's title, although the phrase had been used before by James Stirling in 1857.