On a slide valve steam chest, you need positive pressure in the steam chest to hold the valve down against its seat to achieve back pressure and retardation of movement at all, don't you?

dave

I can see that the greatest deceleration adhesion would be achieved by applying the greatest amount of resistance to the drivers without causing them to lock up. If they do lock up, not only does the deceleration adhesion drop, but also the wheels will get flat spots as they skid.

However, I wonder what happens to the deceleration adhesion if the drivers slowly rotate in reverse to the direction of locomotive travel. They won’t get flat spots under that circumstance. It also seems like the deceleration adhesion would be somewhat higher than a locked up skid, but I don’t know for sure. A counter-rotating driver is always presenting new surface to the sliding contact with the rail. That might produce more friction than the sliding contact of a locked wheel skid in which the contact surfaces remain unchanged.

I can understand why today’s preservationist would not want to stress a locomotive by reversing against the forward movement, but I wonder about historical practice on locomotives without power brakes. When it is said that engineers reversed their locomotive in order to stop, I always assumed that meant that they literally powered the drivers in reverse to the direction of locomotive travel rather than just placing the cutoff into reverse valve motion and producing back pressure to the forward travel.