I think HO trains are designed for 18" radius curves since that was the most commonly used sectional track radius throughout much of the history of HO and HO packaged sets, and many modelers still use those curves.
2 rail O scale had no such sectional track history nor requirement, so minimum radius was solely related to lead and trail truck clearance and swing, side-to-side play in drivers, amount of firebox piping and detail that would impede swing, etc., etc.
3 rail proves that you can get almost any engine around any curve if you take enough liberties with realism, for example, swinging pilots, reduced size of pilot wheels, complete lack of underframe detail in the lead and trail truck area, lack of detail parts that could impede swing, use of blind drivers, truck mounting of couplers vs. body mounting, etc. etc.
While not resorting to the extreme compromises in O gauge 3 rail, many HO manufacturers take just enough liberties to get their engines around 18" radius curves.
The minimum radius of a 2 rail O scale model is also largely determined by how few of these compromises a manufacturer is willing to make, and what the typical consumer is willing to accept. Most 2 rail O scalers seem to prefer fewer liberties with accuracy, and have come to accept the fact that their locos will, therefore, require large radius curves.
Jim
