Another Y-6b detail question

[NW Modeling List]

"There is a box underneth the enginers side running board, above the front
engine, on some N&W Y-6b's. I assume it is a train control box, but don't
know why I see it on some pictures of the engine, but not others."
Dave,
    It is train control/cab signal box and those engines so equipped were 
used mostly on the Shenandoah Div. from Roanoke to Hagerstown. Not all locos 
were used on this line and that is why some have them and some don't.
Jimmy Lisle