1.NounA rail transport system with trains used to transport commuters between two places, typically their homes and works, normally within a metropolitan area and its surrounding environs, and generally distinguished from other rail transport systems (i.e. subways, metros, tramways) by having less frequent service, lacking free transfers between trains and using heavy rolling stock running on a mainline railway"Boston and Philadelphia's commuter rails each have thirteen active lines, but Boston's lines reach, on average, over a third again farther out from the central city than their counterparts in Philly.; I took the commuter rail in to work today, since it's quicker and more convenient than driving on the highway at rush hour."