Etymological Note: This term derives from the tendency of rats to create routes that are out of sight as much as possible, following perimeters and the bases of obstructions.
Citations:
1950 Bob Deindorfer @ Vienna, Austria Los Angeles Times (Oct. 15) “Escapes For Sale” p. G9: I remember a ratline operator helping my nephew escape just after Christmas for two thousand dollars. 1982 William Safire New York Times (Dec. 17) “Did Andropov plot to intimidate Vatican?” p. P7: The Bulgarian secret service, which follows KGB orders without question and which had long-established “rat lines” smuggling spies and arms into Italy and Turkey, was the obvious choice. 1983Globe and Mail (Toronto, Can.) (July 5) “U.S. agents felt ‘no great pangs’ after helping Barbie flee Europe” p. P11: The former CIC colonel said hundreds of Nazis were brought out of Germany by the same route and “everybody that went through the Rat Line was personally accompanied to the ship by a member of our organization.” 2004 Richard Lloyd Parry @ Basra, Iraq Scotsman (Scotland) (Oct. 31) “Shells Land Near Black Watch Base”: The British troops will be supported by US helicopters and unmanned reconnaissance drones which will scan the desert looking for tracks indicating “rat lines”—military jargon for the routes used by the resistance to transport fighters, arms and money.