Here is an update. I wrote to one of the vacuum elevator manufacturers asking how much time in seconds, it takes for the lift cage to start moving after the button is pressed, she replied that it was considerably less than a second. Does this change anything ?
No. The air brakes on a truck or the vacuum brakes on a train work instantaneously (we hope) when required to do so, but it takes time to charge (or evacuate) the reservoir between applications of the brakes.
Why burn the coal at all, why not just place it on the piston and see it do its work !
Because when the piston reaches the bottom of its stroke, you have to schlep it all up the hill again.