first its important to know that all coasting drives are not designed the same. some use a mechanism that disengages the gears and others rely on back driving the worm and gear. I have a car and locomotive shop steam engine that had a coasting drive. the coasting feature worked by the axle gear back driving the worm. this design is accomplished by have a worm that has a double thread. the helix angle of the worm is such that the axle gear can easily back drive (rotate) the worm. the performance of the engine was compromised by a very high gear ratio which made for jack rabbit starts and poor slow speed operation. as far a down hill run-aways, the engine by itself would not run away down hill on a 3% grade but if you added cars behind the engine it would. the only way to control it was to put the throttle in reverse and use it like a brake. this was with DC control not DCC. I did not like this arrangement and defeated it by replacing the motor with a gear head motor that could not be back driven. this solved the run away issue and give me a much lower over all gear ration resulting in much improved slow speed control. i think that coasting drives was mainly a gimmick whos time has come and gone.
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