After seeing some tight curves on an On30 layout at Southampton, a little test track was in order for my Father and I's own model railway layout in that scale.
He found a bit of Peco flexi-track and bent it to a 17 inch diameter curve, held in place with drawing pins to a softboard sheet. The width of curve was forced on us by the width of the wood, not some desire to get locos around corners.
The test wagon ran OK so we powered it up with some croc clips and an old H&M controller. Amazingly, the locos all seemed to run fine. Even the Porter pictured which I worried about thanks to it's long wheelbase if you include the pony truck.
The only thing we have that doesn't like this curve is a Bachmann Shay and even this should be placated by an extra inch or so (as the actress said to the bishop).
OK, so the couplings won't join up on the curves but that won't be a problem. Better still, looking at the plan, we won't need curves this tight after all. Still, it's nice to know.