Paul,
That is difficult to answer because different importers build to different specs. Consider that a 4-8-8-4 in three rail is basically a 2 coupled 4-8-0. For an importer such as MTH or Lionel or 3rd Rail, I'd expect that to make 60' radius easily. The same goes for the Challenger. The problem would come from importers such as Precision Scale, Overland and others who build to tighter specs and include tail beams. The larger wheelbase engines such as a 2-10-4 or a UP 9000 would be more of a problem from the 3 rail importers.
