The Underground Railroad was an attempt to help rogue telepaths escape the Psi Corps. It was named and modelled after a similar effort aimed at helping African American slaves escape the South from the late 18th century to the American Civil War.
From late 2258 to mid 2259, part of the railroad ran through Babylon 5 and was orchestrated by Doctor Stephen Franklin.[1]
Some from the underground helped Lyta Alexander escape the Psi Corps and brought her to Mars.[2]