Despite a fair number of teasers already released, the new trailer for “The Underground Railroad,” from director Barry Jenkins, is the best look yet at the overall plot of his upcoming Amazon Prime series. The show is based on the acclaimed novel from Colson Whitehead and tells the story of a woman, Cora Randall, who is desperate to find freedom in the antebellum South and attempts to escape a Georgia plantation on a mission to find the rumored Underground Railroad.