I'm not big on steam locomotives, but for what's it worth:
In the 1860s, most locomotives were Americans, or 4-4-0. This means they had four lead wheels, four driving wheels, and no trailing wheels. Locomotives were still frequently made by the railroad that would end up owning them. This was when mass production was beginning. The Shay locomotive, which were geared and designed for the type of railroading that tends to be narrow gauge, wasn't developed until around 1880.
One web site where you probably can find people who really know this stuff is MODEL RAILROADER. It also publishes TRAINS magazine.
Narrow guage railroads were used primarily for logging and mining. The two states that pop into my mind are Maine and Colorado, but I know there a lot others. I know Maine still has a narrow gauge running as a tourist railroad.
There are numerous books out on narrow gauge railroading. I'd get one on a railroad that meets your requirements and read it. I think you'll find the answer to your questions.
Jim Clark-Dawe