bridge line

 A bridge line or bridge route was an American rail carrier tasked primarily with moving traffic from one major carrier to another (hence the "bridge" moniker). Bridge lines often were located between two major cities, connecting rail carriers that served those cities and interchanging their cars. As railroads have continued to evolve and large Class I railroads have sought to keep cars on line (as well as collect the revenues for the tonnage moved over their own route miles), most bridge lines are now gone.