Add documentation

Author: peternewell

Source/Documentation/Manual/physics.rst

@@ -2139,6 +2139,10 @@ as the compressor and main air reservoir, and some of the diesel effects (as it
 Some of the cab monitoring gauges provide visibility of what is happening on the powered car. To do this OR searches for 
 the "closest" powered car near the Control car and uses its information.
 
+If the Control Car is "linked" with a geared power car then it will be necessary for the control car to have a gear box 
+controller. To get OR to set it up, it will be necessary for the ``GearBoxControllerNumberofGears ( x )``, where x = number 
+of gears in the associated power car, to be added to the engine section of the ENG file.
+
 
 Multiple Units of Locomotives in Same Consist
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