What kind of magnetic fields would be induced with superconducting wires/rails carrying an AC current? Or is there even any reason to use AC with superconductive transmission?

Those ultra fast and efficient trains required superconducting rails IIRC (which I think indicates strong magnetic field because they used magnets to levitate so that the only friction involved came from air). I wonder if we could combine the trains with transmission and basically have trains that use the power mains to get from A to B.