To start a train. The engineer first moves the quadrant to either forward or reverse, then opens the throttle. Like a manual shift car. Select first then gas. As your going down the track and you want to go faster, move the quadrant a little further. Once you are at desired speed, move the throttle valve to almost closed. Your still admitting steam but not the volume to go faster. Closing the throttle valve is like taking your foot off the gas while in gear but no braking pressure from the engine. Putting the quadrant in the center with the throttle closed is essentially like neutral. Opening the throttle with the quadrant in center can blow steam into the smoke box because of leaking cylinder valves. Not a good idea.