[Suggestion] Add Physical/Friction Brake feature for specific notches + the deceleration values for different brake notches

[DBAG423]

I would like to see an addition to train.dat that allows you to assign friction/blended braking for specific brake notches on trains. This would be useful for physics on trains like some NYCT Subway Stock, where the "Snow Brake" is (on some trains) the 'Min Brake' position and only applies the friction brakes.

A feature that may complement this suggestion is specific deceleration values for the different brake notches, similar to how they are currently used for acceleration.