A dream is a succession of images, ideas, emotions, and sensations that usually occur involuntarily in the mind during certain stages of sleep. The content and purpose of dreams are not fully understood, although they have been a topic of scientific, philosophical and religious interest throughout recorded history. Wikipedia
So this question doesn't seem to be complete since there isn't any answers to choose from but my guess that the author thinks the underground railroad was really cool or served many helpful purposes or something like that. It wouldn't be something negative so whichever one fits most into context.
