Hal, I haven't, but LEDs and light sensors offer a possibility. As explained in a threads a few years ago, I have converted all my operating cars to use light sensors rather than slide shoes. They only open when the car is properly positioned at the coal or log dock. I am confident that the tolerance could be tightened up,
I still can't figure out why the brass shaft of my Bowser would have gotten very tight in the steel bushing.
The smaller 5/16" shaft would bend, Diamond scale used a larger 7/16" shaft/bushing. I had this shaft and horizontal drive bar fabricated. 
Bowser Pit rail boggies were not position to roll smoothly on the pit ring. Note: the pit rail/boggies/wheel alignment. 
I could understand the clamps swelling and reducing the ID of the shaft holes, since they seem to have been zinc, but the 5/16" brass shaft and matching steel bushing shouldn't change. Right now my plan is to make new shaft to table/disc clamps out of aluminum, drill through them and the new 5/16" shaft (aluminum preferred over steel for ease of drilling), and use roll pins. Shaft should be at least 7/16" steel. IMO. Diamond Scale has a 7/16" thrust bushing. The pit rail is anything but perfectly level, even with my fabricated TT. As the bridge turns, with a strait, or slightly bent shaft, rigidly attached to the bridge, there are points where it will bind/stop/ or at least operate slower. Floating attachment is a big plus. 
