metro vs monorailroad

metro

• A rapid transit rail transport system, or a train in such systems, generally underground and serving a metropolitan area. 

• An urban rapid transit rail transport system, normally with lighter track and light trains (i.e. an S-Bahn or similar), or a train in such systems. 

• A metropolitan area. 

• metropolitan 

monorailroad

• Synonym of monorail (“railroad system where trains run on one rail”)