Blp,
I am assuming you are using track with two rails instead of three rails. If that's the case, you also have two lead wires that come from your transformer. You probably have a clip for attaching the wires to the track: use it if you have it. If not, one wire goes to the left rail, the other to the right rail. Secure them to the rails, set your engine on the track and you're in business. You do not need a full circle of track to complete your circuit. Your circle is complete because one wire brings your power from the transformer to rail #1. Your power then enters one side of the train engine through the metal wheels that rest on track #1 and goes to the electric motor. On the other side of the motor is a wire that leads to the wheels that rest on rail #2. From there, the flow is through those wheels to rail #2 and back through the second wire to the transformer, again. Circuit complete.
What you want to avoid is anything creating a short circuit by resting across both rails: a screwdriver, Christmas tinsel, or a carelessly-placed engine. Derailing can cause that last part.