I don't know all of the details...
But on Raptor and Mantis at least, and I think Mean Streak as well, at least at the mid-course, there are prox switches that monitor the train's progres through the brakes and adjust the braking according to the vehicle speed, so that the train speed should be reasonably constant coming out of the brakes.
But there is more to it than that. I have been told that the trimming program on Mean Streak takes into account the train's speed coming into the brakes, the air temperature, and the wind speed and direction as well. So there is some kind of mysterious formula used for figuring out how much the ride will be trimmed.
On Magnum, the trim brakes don't have any associated hardware. There are some calipers there, with air lines feeding them. No switches, no relays, no nothing. I don't know if the braking pressure is determined by the PLC or by an operator with a regulator knob. I do know that on consecutive rides you can get different degrees of braking on Magnum, and it does vary with wind speed and direction. But then, the trims on Magnum are just that; they are not block brakes.
On Gemini, I believe it is more like the old system on Mine Ride, where the brakes are set for block-stop, trim, or off. I can tell you from experience that the trims on the block brake make very little difference in the ride from there to the helix, but they make a *huge* difference in the ride through the helix. Gemini has run a lot in recent years with the trims off in an effort to make sure it completes the circuit.
--Dave Althoff, Jr.