Roland and his ka-tet encounter Blaine, the insane bullet train, in the ruins of Lud.
Lud is a devastated city in Mid-World, divided between rival groups known as the Pubes and the Grays. Roland, Jake, Eddie, Susannah, and Oy enter the city while following the Path of the Beam. Jake is captured by Gasher, forcing Roland to pursue him through Lud's dangerous ruins.
Meanwhile, Eddie and Susannah reach the Cradle of Lud, where Blaine the Mono lies dormant. Blaine is both a highly advanced computer and a monorail train with a murderous personality. He offers the ka-tet passage out of Lud, but only if they participate in a life-or-death riddle contest.
Lud is sometimes confused with New York City because of the series' many connections between worlds, but New York is not the answer to this question. Calla is another location in the Dark Tower universe, while Omaha is unrelated to this encounter.