If you get confused about what's happening, refer back to this diagram for an illustration.
The above diagram from the site http://www.wtec.org/loyola/scpa/09_12.htm illustrates the following three systems.
How can a huge train levitate?
Trains propelled by electrodynamic suspension system levitate over the track with a distance of about 8-10 cm while the electromagnetic system only hovers at 1-2cm above the track. The levitation in the electrodynamic system depends upon induced currents caused by the movement of the train's electromagnets and the levitation coils on both sides of the track. Remember when a magnetic field changes over time, an induced current is created opposing the changing magnetic field. The train is moving, so the magnetic field into the sides of the track changes as it crosses each levitation coil, always creating an induced current.
The train does not levitate until a velocity of about 100 km/hr is reached, and at that point the change in magnetic flux in the levitation coils causes an upward magnetic force strong enough to counteract the force of gravity and levitate the train. The train has wheels so it can move until levitation takes over.
The levitation coils use a special null flux design, which is basically two square coils of wire very close to each other at one point, but not actually touching. The magnetic field from the train points into the bottom half of the levitation coil, so the induced current will create a magnetic field equal and opposite in magnitude opposing the train on the bottom levitation coil. Look at the top diagram, and this mean that on one side the current flows clockwise, creating two north poles are now opposing each other. The top coil is close enough to the bottom that the current through the bottom creates a new current, and by Lenz's law an the new induced current opposes the current that created it, so the top coil's current flows in the opposite direction, counterclockwise. Since this current flows in the opposite direction, the magnetic field coming out of the coils is opposite to the other two fields, south.
Now there is an attractive magnetic field that attracts the electromagnet on the train upwards and a repelling magnetic field at the bottom that also propels the electromagnet upwards until the train's electromagnet reaches the midpoint of the coil. The special null flux design keeps the train levitating at the midpoint of the two coils and also helps reduce the opposing magnetic drag discussed in propulsion. For a longer explanation of exactly how the geometry of the levitation coil reduces flux when the electromagnet is at the midpoint of the two coils, go to this site: http://www.public.iastate.edu/~dstieler/maglev.pdf
Once at the midpoint and while moving with a sufficient velocity, any time the train moves up or down an induced emf opposes that change and returns the electromagnets to the midpoint of the two coils.
So, since there's an attractive, opposite magnetic field on the top half of the levitation coils, why doesn't the train get stuck on the side of the track?
Good question! The reason the train doesn't get stuck on the side of the track is, once again...(just guess)...because of Lenz's law and induced currents! Guidance, the name for this induced current, opposes sideways motion of the train and keeps the train in the middle of the track.
Guidance coils may be the levitation coils, or located underneath the levitation coils. Since the electromagnet is a solenoid, recall the intensity is largest very close to the center of the solenoid. When the electromagnet moves close to one side of the track, an induced current occurs because the intensity of the magnetic field increases. An induced current from the guidance coils is created on both sides of the train. If the train is too close to one side of the track, the train is repelled on that side and on the opposite side of the train the guidance coils exerts an attractive force attracting the train back to its previous position. These two restoring forces push the train back to the center of the track, causing the train to remain a constant distance in the middle of the track.
How does the train move forward?
The propulsion system moves the train forward, and is different from the levitation and guidance system. The propulsion coils in the track have an ac current that propagates down the track in front of the train. This alternating current creates an opposite magnetic field to the electromagnet, and since opposite poles attract, the electromagnet pulls the train moves forward. The alternating current also creates a repulsive force at the back of the electromagnet, helping push the train forward. Since the ac current changes directions, once the train reaches the propulsion coil generating the attractive magnetic field, when the train's propulsion electromagnet is at about the halfway point of the coil the current changes, so now the train is repelled at that spot and pulled forward to the next propulsion coil, a cycle that repeats down the track. Since the electrical current propagating down the track use AC current and the electromagnets on the train also use AC current, calculating of the needed current to attract the train as it accelerates are complex.
Opposing forces to the forward movement are induced currents causing magnetic drag and air resistance. An induced currents oppose the forwards motion of the train and will eventually help slow the train down to a constant speed, along with the air resistance which becomes significant at high speeds. (Notice that when the train levitates, no friction occurs between train and track, a huge advantage from the traditional train!
Other Factors to Consider regarding EDS trains