Meaning: This sentence describes an event where a series of connected railway carriages moves from one place to another across or within large natural elevations of the earth's surface.
Choice of Words: The train is a noun referring to a series of connected railway carriages. The verb traveled describes the action of moving from one place to another. Through is a preposition indicating movement across or within something. The mountains are nouns referring to large natural elevations of the earth's surface. The word the is used to specify a particular train and mountains.