Good answers. I think we have to give this to BB Joe. He is pretty close with his answer and was first. As the engineer explained to me. There are a number of ports along the cylinder. A slide rail determines which port the steam enteres the cylinder through. When the locamotive starts out the engineer sets the control so the steam enters at the far end of the cylinder thus pushing the cylinder for the full length of the stroke. Once the train is rolling the lever is moved allowing the steam to enter with the piston having already traveled part way down the cylinder. Hence you get full effect of the steam on start up and only a short burst of steam when you are cruising. Part of the skill of an experienced engineer is coordinating the two controls to optimize the use of fire and water for work performed. So you can see the search for fuel economy is not new. This technology just fascinates me.......How can a few gallons of water and some firewood move that much weight down the tracks?
Your floor BB