When I was designing an Arduino based 'shuttle solution for BIL's OO layout [1], we found just how different the motors were over the ages re their power requirements.
Using PWM, if we found a low level setting that might just allow an older loco to roll along slowly but reliably, the same setting would have a new loco going along at quite a rate!
So long / short, I think we concluded that rather than going for the ultimate goal of being able to put any loco on the (automated) track and expect it to work equally, we would focus on just a few, maybe even just a railcar or a similar self-contained unit and on an outer circuit away from all the other / analogue stuff.
Cheers, T i m
[1] The idea was to have a single track run around his layout that terminated in two sidings at each end. Each 'section' of track (4 sidings and the main run in maybe 3 chunks) to have occupancy sensing via current and positioning via encoded optical (reflection) sensors.The idea is that he could have up to 3 locos on the layout at any time and in any siding on the cct at startup. The controller would test for occupancy by polling all the sidings and then randomly select one of the two at the double end and set the points to take it to the empty siding at the other. The speed on the main run would also be managed (with inertia) via feedback from the occupancy sensors with the possibility of a halt / station in the middle somewhere.
We got as far as a short length of test track with a bypass in the middle and a loco starting at one end, being taken one way round the bypass on the way up and the other on the way back, accelerating and decelerating as it did so using the optical sensors near each end (one to start the train slowing as it approached the end and another to bring it to a full stop).
We stopped working on it because he moved and he's currently re-building his layout in the (now well insulated) garage, rather than loft. ;-)
The idea was to give him something to have running in the background whilst he's doing other stuff on the main layout and I know you can buy commercial solutions to do this, part of it was the fun of doing it oneself. ;-)
I was thinking of getting myself a couple of N gauge railcars and setting up a similar system up on the walls around a room here.