To me it seems that narrow gauge cars have the cut levers on the right side of the A end of the cars. This is also the same side the engineer is on and on a short train this makes it easyer for the engineer to see the brakeman. Thats seems to be the most likely/simple reason to me.
When switching at Knotts the barakeman is always on the engineers side. And in most cases I(the engineer) can see him. Sinse we only run one direction we moved all the cut levers to the right side of the cars.(well the passenger cars have levers on both sides as they always did) So that way when uncoupling a car the brakeman is always on the engineers side.