Help:DCC Dial
From TrainzOnline
This dial controls the movement of the currently selected train and is analogous to a model railway DCC controller knob. Click LMB and drag it clockwise on the dial to move forward. Dragging the dial so the arrow points upwards will stop the train. Dragging the dial anti-clockwise will move the train in reverse. Clicking LMB on a position of the dial will also move the dial to that location.
