On the Kingda Ka animated video i have seen, Just before the launch bit a small plate like thing slides under the train. Is this a magnet that is pulled to the end of the launch track and by then the train has got enough speed to make it up the tower. I was just wondering if this is how those motors work could somebody please tell me exactly

That thing is the carriage that attatches to the train, and is pulled along by cables powered by a hydraulic motors to launch the train. Kingda Ka is not powered by Linear induction motors, they have been superseded by the hydraulic launch system. The only rides that really use LIMs are smaller scale shuttle coasters like the Intamin Impulsa and the ride trade Half Pipe coaster.

KK like TTD and Xcelerator is not a LIM/LSM launch coaster. It is a Intamin Rocket (hydralic launch) coaster...that is it is launched by being attached to a steel cable which is pulled by a high speed winch to launch the train. The car you saw is the means by which the train is attached to the cable. This catch car couples to the underside of the train around the middle and then when the train gains enough speed it over takes the sled and continues around the track. I know this doesn't answer your LIM question but this is how KK will work.

Yeah, thanks i was kind of just wondering how those things were launched