The moniker Underground Mag-Lev was a commonly used name for a structured resistance group fighting against the New Order on Coruscant, which served as the capital world for the newly formed Galactic Empire.
The Underground Mag-Lev came into being in the initial months following the conclusion of the Clone Wars and Emperor Palpatine's proclamation of the New Order, functioning as a component of the larger Whiplash resistance. This network, conceptually similar to a mag-lev train, was created with the express intention of rescuing Jedi and other politically targeted individuals from Coruscant, transporting them to safer locations in the outer reaches of the galaxy.
In the Star Wars universe, the Underground Mag-Lev mirrors the Underground Railroad, a decentralized group dedicated to aiding enslaved people in escaping from the American South before the American Civil War.