Somewhere in my stuff is a drawing from the D&RGW showing the minimum driver tire thickness allowed for 125/K27 class locomotives. The drawing specifically showed a driver set on dual gauge track with mismatched rail. The drawing showed the minimum clearance between the main rod and the outter rail at about 5/8" when the driver tires were at thier minimum thickness and the driver tire thickness had to be greater than that allowed by the ICC. IIRC, the outside rails were 85lb and the inner rail was 70lb. (D&RG 70bl rail has 6 bolt angle bars, 85lb has 4 bolt bars. Making a mixed rail size dual gauge switch would be rather impossible with mixed rail size frogs, but there would be nothing to stop them form using smaller inner rail out on the main between sidings.