Help:DCC Stop
From TrainzOnline
Stop (S)
To quickly reset the dial and bring the current consist to a halt, Click LMB on the Stop icon. You can also use the “W” and “X” keys for forward and reverse controls and the “S” key for stop. If you are using the keyboard to control the throttle, you can refer to the position of the rotary dial on the HUD to see your current throttle setting.