@A. Wells The 4 detents per rev hasn't been a big issue. At it's slowest, my engine axles are doing about 90 rpms. So a magnet goes past my Hall detector about every 160ms. And I run a low-pass filter over the measured rpms before using them. If the train is on straight track, the measured rpms stay pretty close to constant.
My biggest issue right now is I'm stretching the limits of the gearing/traction of these particular powered trucks. I'd love to run even slower, but then bends in the track or other random friction can cause the can motors to come to a halt. (And faster than even a PID can compensate for.)
One of my next projects is to put my boards in an RMT Beep. The low-speed gearing on those is spectacular. (But I'll have to do some miniaturization on my protoboard. Very limited space inside a Beep. I might not be able to fit a tachometer inside a Beep either. But again, their low gearing might make a PID controller unnecessary.)
