Antinos said:
I'm fairly certain Jolash hit the nail on the head. Coasters like Gatekeeper, Diamondback, Leviathan, Maverick, and other modern coasters with magnetic brakes tend to have a combo of brake fins (some static, some retractable) and drive wheels. The wheels are used to stop the train in these cases.
Regarding the impulse coasters. I'm almost positive (no pun intended) that they just switch the direction of the current until the train comes to a stop.
Bingo.
A 100% magnetic braking system does not actually exist on any full circuit roller coaster. Instead, "hybrid" would be a better term, using a combination of magnetic brakes for the initial deceleration, and frictional brakes (friction wheels, friction brakes) for the final stop.
For instance, majority of B&M brake runs use a horizontal set of magnetic brakes at the top of the brake run, followed by their traditional friction brake, which stand vertical.
To get a good look at the braking hardware on-board, wing coasters offer a great, open view of the assembly.
For the magnetic brakes, notice the horizontal brake fins on the train.
The friction brakes meanwhile catch onto the center spine of the train, which carries some metal/carbon compound. You can see the signs of friction in the center of the train, riding just over the tracks.
Intamin meanwhile begins with magnetic brake fins, slows trains to a very slow pace, and uses friction tires for the final stop.
For impulse roller coasters, it is in essence throwing the roller coaster into "reverse" to bring the train to a stop. The LIMs are then used to bring the roller coaster brake into the station, and taper off once the train is in position - it is an admittedly slow stopping procedure vs. full circuit coasters.