Maglev Train Propulsion System
Each electromagnet has an H-bridge circuit. Reverses polarity the same way a brushed motor reverses direction.

Understand H-Bridge

All the circuits are coordinated with a processor or something like that. Find a more manual route for a science fair project. You probably don't have enough time or money to learn and build a processor controlled version of anything.

Are your electromagnets on the track or train?

If on the track, you are going to need a LOT of relays wired up as H-bridges. Four SPST relays or two DPDT relays per electromagnet. Definitely not enough time to build a loop this way even if you had 100% of the knowledge already required. If this is what you plan then demonstrate just a short linear section. I don't think you can afford the money or time for all the relays for a full loop.

If on the train, you could use permanent magnets for the track and put the electromagnets on the train. Less parts but your train is heavier and harder to move. You could use hall switches to detect the permanent magnets to decide when to switch the electromagnets, but you definitely need to get help, in person, from an adult who already knows electronics since circuit design and soldering will be involved. Alternatively draw black/white marks under the track that the train views to tell the train when to switch polarities. Might be easier since this is more similar to the simplest line following robot designs which you can copy. You could do this without a processor involves a more complicated circuit, but it might be a bit easier with a processor if you pick one super, super easy to learn like a PICAXE or Arduino.

I used to judge science fairs and having anything functional that works properly is impressive. Even manual switches that you flip in the proper sequence to inch the train along would impress.