As the current flows through the coil, it creates a magnetic field, which opposes the magnet in the front of the train (the two green Ns) so it pushes the magnet away. At the rear of the train the opposite happens. The South end of the field attracts the North end of the magnet and pulls the train forward. So the train gets two forces speeding it around the track: the pushing force in the front and the pulling force in the rear