You can't patent a unit of measure. What the MTH patent (6,619,594) claims, among other things, is a pretty broad implementation of speed control:
5. A model train which receives commands in the form of data bit sequences, comprising: a processor which receives one of said commands corresponding to a desired speed of said train; a motor control circuit; and a speed control circuit that monitors the train's speed and provides information to the processor concerning a current speed of the train, such that the processor compares the current speed of the train to the desired speed and outputs a command to a motor control circuit to drive the train to run at the desired speed.
And a dependent (narrower) claim which specifies the same speed control system but having the additional feature of being controllable in SMPH:
12. The model train of claim 5 wherein the model train's speed is controllable in 1 scale mile-per-hour increments.
(There are some similar generic claims with a dependent claim in the same patent).
The "patent on SMPH" issue is a red herring. If the accused control system had the generic features recited by claim 5, then it would infringe the patent whether or not it had SMPH.
The only circumstance in which the dependent claim would be significant would be if the broader claim was invalidated. E.g., the broadly defined speed control system of claim 5 is found to be obvious over the prior art, but a speed control system additionally including the SMPH feature was non-obvious. In this case the patent would only cover a speed control system with the SMPH feature.
The validity of these claims is rather questionable in light of the Supreme Court's 2007 decision in KSR Int'l Co. v. Teleflex, Inc.
