Bob,
Your are correct about making the drivers wider for 3 rail track. If a 1/4" scale steam loco was manufactured correctly as a 2 rail model. there will be no room to fit wider 3 rail tires on the drivers. Their will be interference between the side rods and the crossheads. Making things smaller when converting 3 rail to 2 rail is not a problem. Not so in reverse. I would not even attempt it. And if the loco is converted simply by installing three rail rollers I do not think the scale drivers will work to well on hi rail switches. No one will be happy in the end.
in my experience with 2 rail conversions of Lionel and MTH steam models, I find that the hi rail drivers are usually about 0.230" wide front to rear of flange. I machine my 2 rail drivers to 0.160" width, that is 0.070" machined off of the driver casting width, some from the front edge and most from the rear of the driver center. This is done with the new rough tire blank pressed onto the driver center and held in a turning mandrel held in a precision collet checking axle run out with a Last Word dial indicator. I can normally hold about 0.001" concentricity on my drivers.
I am currently finishing up the conversion of a MTH N&W Y-3 and Y-6b. There is considerable difference in the mechanisms of these locos. Y-3 is finished and operational and I hope to finish the Y-6b today.
Joe Foehrkolb
