In TrainPlayer, a train is defined as any contiguous set of cars, whether moving or not, with or without motive power -- a single car sitting on a siding is considered a train. However, a train will not move unless it has an engine. To move an engineless set of cars, you must either couple them to a powered train, or use "artificial means" equivalent to a railroader's hand reaching across the tracks.
You may have any number of trains on your layout, all running simultaneously if you can manage it.
Most of the layouts supplied with TrainPlayer come equipped with one or more trains. You don't have to settle for our selection -- you can delete or modify the provided ones and/or add your own; see the chapter on "Building Trains." In this chapter, we'll work with the train which comes with the Turtle Creek Central.