What is the importance of the title, Underground Airlines?

The use of the term "airline" instead of "railroad" symbolizes the historical differences between the two time frames and their mutual connection.

During the early nineteenth century, the "Underground Railroad" referred to a group of abolitionists who provided a series of safe houses and secret routes to help escaped slaves flee the United States. Like the “Underground Railroad” groups of the nineteenth century, Underground Airlines help to hide and smuggle escaped slaves to areas where they cannot be legally recaptured.