The wagon has an optical counter, counting an axles rotational speed. When the train slows down, the regulator is opened a notch, the controller waits to see if speed picks up, if not then the regulator is opened up another notch. When the train is above speed the regulator returns to it's running position. Then afterwards if the train is still over speed then the regulator closes one servo step per second, until finally the train is under speed. And, so on.
The parameters that I have used are suited to my line. My line has quite large radius curves and gentle grades, except for the final 180 degree return curve, but that's downhill.
Here's my best video showing a circuit of my layout, with a heavy test train.
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The construction of the R/C control, automatic speed control is detailed here:
http://www.santacruzlumberco.com/MLS_PD ... Steam.pdf"