That name was a sadistic play on the Underground Railroad that smuggled American slaves north. The old Nazis set up their own version and used it mainly to move their people. They called it Die Spinne.

Dear Artie: “The young fellow has disappeared into a dead end. I think the long-necked bastard planned to wind up in Paris and sent him there but he may also have used the underground railroad. Ask your round-heeled contact. Maybe you can find more than I could. “Roy