Minimum radii

The prewar standard gauge locos and cars was designed to operate on 42" diameter track, altho the State cars will look much better 72" diameter track.

Ron M

Frank, Ron about said all there is to say on this, there are a few modern standard gauge pieces (Super 381, Brute & its cars, probably some other big engines by Liberty Lines or Roberts) that won't make it around 42 curves, but otherwise, all the traditional standard gauge equipment (and reproductions of same) will do just fine.  The bigger cars like the State set are long enough that they have a lot of inhang going around 42's, but if you're into standard gauge tinplate, realism probably isn't too much of a stickling point.  To most people, these big cars look better on 72's or 84's, but it's strictly aesthetic, there is no operational issue.  And any of the other equipment, like the flyer presidential cars and so on, really are fine on 42's.

One trick if you don't have room for pure 72 or 84 curves, is to transition in and out of a tighter curve.  Instead of four 72 curve pieces to make a 90 degree turn, go with one 72, then a 42, then a 72.  The train eases in and out of the sharper curve and doesn't look so much like a toy train as it might going around a 90 degree corner made of two 42's.

Transitioning into the 42 curve with a section of wide radius track is an excellent and proven method for improving appearance and function.  Keep in mind some of the "modern" Standard Gauge (including the Lionel Hiawatha and Commodore Vanderbilt) requires 72 curves as a minimum, and everything looks better on 84 and 87.