You can remove the GW-180 controller and just use the brick. Add in a few properly wired bridge rectifiers to drop the track power to 11-12 volts and you have a permanent speed limiter. If you make a small module with the bridges in with connectors that only work one way, they can't take the protection out.
If you jumper the +/- pins and then put the ~ pins in series with the line, each bridge will drop about 1.5 volts. To go from 18V to 12V you'll need four bridge rectifiers, probably 3A ones would be plenty.
