Westside models made a heisler with O and On3 trucks.
Third rail pickups and bigger flanged wheels could be added if wanted.
They can be converted back by switching trucks.
They are easily found used on EBAY.
Here is a photo of one, much prettier than mine.
Heisler with O scale trucks installed, On3 trucks loose.
(not my Heisler )
My Heisler set up for On3