> Was the M.P. value a genuine decimal figure? If so, how was it physically measured?
It is not all that hard to do. Just measure the distance from the milepost using multiple measurements of a long tape or standard surveying chains. Railroad Civil engineers measured accurate distances all the time. Once you have the number of feet from the milepost just divide that by 5280 to come up with the decimal equivalent. A school child can do it.
When I was injured in the 1990s and on "light duty" my assignment was to go out on the road each day to a different siding or junction and measure the precise feet between such things as signals, switch points, clearance points, automobile crossings, etc. The information was to be used to print "guide books" for he train crews so they knew where their trains would fit and where they would not.
To do the actual measuring I was given a calibrated "surveyor's wheel" which was just a spoked wheel about 2 ft in diameter and had a mechanical counter attached to it to count each foot it moved. There were a couple of homemade rail guides on each side to make it easy to keep the wheel on top of the rail as you pushed it along. I walked a LOT of track pushing that thing along the top of the rails. Most of the Q from Gillette, WY to Huntley, MT. Yard tracks too.
With the right counter design and wheel size a similar device could be made to directly readout in 1/100ths or 1/1000ths of a mile. It isn't exactly rocket science.
AK