Please see below two short videos showing the results of a first attempt using electronic to create soft starts and stops for a model, in this instance my Beam Engine. This was just a temporary setup to test the principles of the circuitry.
The first clip shows the Stop/Start switch arrangement and the second clip shows the circuit in operation. This is very much a first iteration to be developed with different component values as necessary to give better results. Any jerkiness seen is because of video editing and not actually present in the motion.