Shuttle train is a commonly used term that refers to a train service that runs regularly between two specific locations. There are several synonyms for shuttle train that can be used interchangeably depending on the context. Some of the most commonly used synonyms include commuter train, express train, rapid transit, metro, and light rail. Each of these synonyms may be used to describe a train service that runs frequently, transports passengers quickly and efficiently, and serves a specific route. Ultimately, the choice of synonym will depend on the specific train service being referred to and the context in which it is used.