Rail transport such as trains, trams, monorails, railway tracks and train stations are recurring themes throughout the Half-Life series. At numerous points in every one of the games (excluding Portal), the player encounters one or more moving or stationary locomotives, lone carriages, or full-length coupled trains. There are also several instances where the player must cross or follow railway tracks, either on foot, non-rail vehicle, riding in or on the train itself, or by use of an electric monorail, the latter a prominent feature in the original Half-Life game.