For example, when trailers containing new automobiles were first piggybacked two areas of potential damage became evident: (1) diesel locomotive exhaust left a film of oil on the new autos; and (2) auto windshields could be scarred or cracked by the metal-tipped "tell-tales" which warn men atop trains of oncoming bridges or tunnels. Accordingly, automobiles aboard piggyback flats are usually coupled into the train 15 or more cars behind the locomotive; and telltales have been raised.