Answer:
No, it was a secret network to the North for escaping slaves in the South prior to the US Civil War. It was a step-by-step passage created by escaped slaves and abolitionists, including sympathizers in the South. So it wasn't actually a railroad, or even under the ground.
It is called "underground" in the same sense as the French Underground of World War II, a secret group within an oppressive society. The term "railroad" is used because, much like trains travelling from station to station, the slaves were moved north over a few days or weeks, usually at night. Anti-slavery sympathizers allowed the use of their houses and farms to feed, clothe, and hide the escapees.