Yes one of the "3rd rails" is +300V DC, while the other is -300V DC. This is because the trains can't discharge the spent current into the rails like non-LIM trains do, due to the wheels being lifted off the tracks while in motion. Its the same priniciple as Trolley Buses having 2 polls and wires since rubber is an insulator, so spent current can't go through the wheels like it does on a streetcar.
I believe not exactly "while in motion" (that would have negated the need for wheels at all) but only initially when the LIM first pushes the train forward. The acceleration forces are forward and up, so for an initial brief second, the wheels leave the track by mere millimeters, but its enough to break the electrical circuit. So what happens is you get a terrible oscillation effect, where the motor pushes the train up, cuts power, motor stops, train falls, contact is reestablished, motor pushes train forward, train lifts off track, power is cut. Over and over.
I'm surprised that since it was only a small brief period where contact is lost, they didnt just opt for a small battery on the train or a large capacitor, to fill in the small gaps when the train is lifted. Seems to me that would have been cheaper than constructing an entire other 4th rail.
