When the train is placed completely within the copper coil, the circuit is closed. A current flows from the +ve end of the battery, round the local part of the

When the train is placed completely within the copper coil, the circuit is closed. A current flows from the +ve end of the battery, round the local part of the coil, back into the -ve end of the battery (the magnets are conducting). This circulating current generates a magnetic field. Proper orientation of the head and tail magnets leads to both magnets being pushed in the same direction because of their interaction with the generated magnetic field. This is the force that drives the train.