A commuter carrier is a type of transportation service that provides transportation for commuters. RAIL, which is a four letter word, is a form of public transport consisting of a series of connected vehicles that run along a track to transport passengers. SHUTTLE, which is a seven letter word, is a BUS or train service that runs between two places at regular intervals. STOL, which is a four letter word, stands for Short Takeoff and Landing and is an aircraft with a short runway requirement for takeoff and landing. Finally, BUS, which is a three letter word, is a large motor vehicle used to carry passengers.