Data on parallel center rail spacing required (minimum) for different locos at different curve diameters

  Using geometry you could figure overhang clearences from any one set of measurements.

    I'm far to rusty to actually do the math myself with confidence, but it should be possible for someone in practice of geometry today. (usefull if a Big Boy owner only has 50" track to measure on)

Actual working min. dia. and tricks for smaller curves and/or noting listed min. would be nice too.

You only have half of the equation.  Assuming the locomotive is on the inside of the curve, what is on the outside?  I have 4.25" center to center spacing on O72 inside curves.  My biggest engines (Lionel Y-3 2-8-8-2 and MTH Centipedes) work fine with a freight train (scale 40 to 50 foot cars) on the outside curve.  The combination of one of these locomotives on the inside with a passenger train (scale 80 foot cars) on the outside results in a sideswipe.  A friend has tested a scale Big Boy in the inside with scale 80 to 84 foot (21 inch) passenger cars on the outside and found that 5.5" center to center spacing was required.

Yep, we determined that we really need at least 5.5" on O99 curves to allow the VL-BB and 21" passenger cars on the adjacent tracks not to get together.  The scale Allegheny, Challenger, and Y6b come in right behind with just a bit less overhang.

On the smaller side of things, RailKing and Lionel semi-scale and smaller scale locomotives typically don't require nearly the same amount of clearance. My scale A-5 hardly hangs over at all.

If you're laying track, it's likely very important to determine the worst case for your current and intended future locomotive and rolling stock mix to determine what clearance you will need.