Jonnyspeed,
My mainline minimums are at 56" and 60" and that works for everything I own. Those minimums were determined by some of my more highly detailed locomotives. In hidden storage areas I have loops that are 60", 56", 51.5", 47", 38" and 34".
I've run the MTH J on 47" track with no problems. I've also run the Overland J on 47" track with no problems. When it gets down to the tighter tracks on my layout (38") the MTH engine will get around it but it is really awkward on there and I wouldn't run it fast on there. I think the engine is rated for 40", so I believe that really is a good rating for it. The drivers want to straighten the track on 34" curves.
I've run the Sunset Y3 and the Y6 engines and the MTH A on 38" track with no problem other than on the Y6 the extended handrail on the back touches the tender. One would have to extend the drawbar or do something with the handrail. All of these tracks are in hidden areas.
I do have a short, 5' or so, 36" radius run of track in a mine area that the Sunset Y3 and the MTH A can negotiate but they really haven't been on that track except for 2 or 3 times in 8 years.
I will say, seeing these big trains on the tightest curves does not look as good but that would apply to 2 rail or 3 rail and H0 I suppose. I think you really should be looking to give yourself the larger radius curves on any visible mainline trackage if you can.
