Bellaire & St. Clairsville
Celina, Van Wert & State Line
Chagrin Falls & Southern
Cleveland, Delphos & St.Louis
Columbus & Maysville
Columbus, Washington & Cincinnati
Connotton Valley
Lake Erie, Alliance & Wheeling
Painesville & Youngstown
I'm not sure where Ohio stood in the ranking of states with the most narrow gauge mileage, but it must be near the top. The T C & St.L, as
Myork has pointed out, was part of an effort to build a large integral narrow gauge system running from the Great Lakes to the Gulf in competition with the standard gauge roads. The Conotton Valley, and the Painesville & Youngstown were well-built heavy-haul railroads operating between the Great Lakes and the interior mining/manufacturing regions.
It's always seemed to me that too many narrow gauge fans focus on California and Colorado to the exclusion of all else and thereby miss much of the history of the narrow gauge era. For example, in addition to the great lakes-to-gulf trunk there was an effort to build a transcontinental narrow-gauge system. The Ohio mileage, chartered as the Washington, Cincinnati & St. Louis was graded but not built. Ultimately, the narrow gauge with it's cheap construction and high operating costs proved to be unable to compete with the standard gauge roads in the long haul. On the other hand, a lot of narrow gauges that were built as local short lines - feeders to the mainline railroads - served their function quite well until the industry they were built to support folded, or they were superceded by paved highways and local truck haulage.
In addition to Ohio, Indiana, Pennsylvania, Georga, Florida and others had extensive narrow gauge mileage at one time or another. There's a lot of history out there that hasn't really recieved the attention from railfans that it deserves.
J