WordNet (r) 3.0 (2006):

trade route
    n 1: a lane at sea that is a regularly used route for vessels
         [syn: seaway, sea lane, ship route, trade route]
    2: a route followed by traders (usually in caravans)

Moby Thesaurus II by Grady Ward, 1.0:

22 Moby Thesaurus words for "trade route":
   air lane, beat, circuit, course, flight path, itinerary, line,
   orbit, path, primrose path, road, round, route, run, sea lane,
   shortcut, tour, track, traject, trajectory, trajet, walk