I had seriously considered going to On30, but my interests lay primarily in portraying the manual interchange of narrow gauge with standard in a limited space.
Advantages of On30, On3, and Sn3 is that they each have sufficient size (slightly larger than HO standard gauge) to have good looking and good running locomotives, even when the prototypes are very small. On30 has over the other 2 a good selection of reasonably priced locomotives and rolling stock.
Most of the On30 locomotives are scale models, except for track gauge, of 3 ft gauge protoypes. 3ft was the predominant narrow gauge in the US. 2ft was a distant 2nd, and the assorted other gauges - 42 inch, 30 inch, and 18 inch all saw some use. The rolling stock from what I have seen is more of a mixture of 2ft, 3ft gauge prototypes, and generic short narrow gauge cars. Your needed clearances on curves, bridges, tunnels, loading platforms, etc is going to depend on what size rolling stock you choose. The smaller cars look a little too small to go behind a K-27 model - but that's my eye. On30 modelers are known for having fun, and not taking exactness in modeling too seriously.
To realistically model On30 in small spaces requires a very good eye for selective compression of structures. This was the deal breaker for me - the size of a realistic structure was just too much for my space.
HOn3 and HOn30, on the other hand, present opportunities for less compressed structures and scenery. However, getting smooth running, slow speed locomotives, especially of small prototypes, is not a given. It can be acheived, but does not always come that way from the box. HOn30 typically start with N chassis for their locomotives. HOn3 has a fair amount specifically made for the gauge.
Most importantly, do what suits you, and is fun for you! It's your railroad!
yours in narrow gauging
