Meaning: This sentence describes a vehicle that runs on rails moving over a structure built over a natural waterway.
Choice of Words: "Train" refers to a vehicle that runs on rails, "travels" describes the movement of the train, "across" indicates the path the train takes, "bridge" is a structure built over a river or other obstacle, and "river" is a natural waterway. The sentence creates a vivid picture of a train moving over a bridge spanning a river.