x1 is a train [vehicle] of cars/units x2 (mass) for rails/system/railroad x3, propelled by x4.
A railed vehicle or train of vehicles; also subway (tu'unre'e), metro, trolley, tramway (= lajre'e), roller coaster; monorail (= dadre'e); cable car, sky car, ski lift (= cildadre'e). See also tcana.