Re: Narrow Guage less than 36"

Hi Wade,
1. Why, in Maine, was 24" used instead of 36"?
Because the 2' gauge idea was promoted by George Mansfield of Massachusetts, who had visited the fully operational ffestiniog Railway in the 1870s. Mr. Mansfield was the "founding father" of two foot gauge in this country and promoted many such lines in this country and elsewhere. Basically, when the Mainers were looking, Mr. Mansfield arrived bag in hand and talked them into it. This effort was made easier by his allusions to fully operational two footers elsewhere and fairly accurate figures that showed these lines were much cheaper to build than standard and three foot lines. In fact, George Hilton states in his book "American Narrow Gauge Railroads", "Edward Hulbert (promoter of the first two US narrow gauge conventions in the 1870's) in 1872 rose to the level of enthusiasm of calling 2'0" railroads "better than going afoot," but by the test of survival, they provided the function of purely local service more effectively than the 3'0" lines he advocated."
Therein lies the answer to the question, "Why weren't more US common carriers built?" By the time the first US common-carrier two footer (the Billerica & Bedford) was built, three foot gauge had already become the "standard" for narrow gauge construction in this country.
2. Was 24" used in other parts of the country as well? I have heard of a couple of two-footers in Colorado, but don't know anything about them.
Yes, however these lines were primarily built to serve particular extractive industries, agricultural operations, utilities, or tourist purposes. With the exception of the fledgling Billerica & Bedford RR (and perhaps the Edaville RR in Massachusetts (which in early years carried both passengers and cranberries)), the State of Maine boasted the only "classic" two foot gauge common carrier operations in the US.
When this topic is discussed however, 3 additional lines are often mentioned. The line in Colorado (prior to the modern-day CC&V or the somewhat recently deceased High Country - which are/were tourist lines) is the Gilpin County Tramway. This line provided transportation for the many gold mine operations in the Blackhawk, CO area. A sister line called the Silver City, Pinos Altos & Mogollon served a silver mining district in New Mexico. And the Mt. Gretna Narrow Gauge was a 4-mile tourist line in Mt. Gretna, PA that rostered 3 Baldwin 4-4-0s around the turn of the last century.
3. Was 36" used at all in Maine?
Yes. Hilton lists a common carrier line called the "Bucksport & Bangor". This line was built before the two foot era dawned.
4. Were there any NG RR's that was less than 24" (that wasn't strictly a tourist line)??
Yes, but in the US these were primarily operations designed to serve industrial, agricultural or extractive industries. If one goes world-wide however, there are examples. The best ones I can think of - and the narrowest ones possible - were the steam monorail lines. One was the "Peg Leg RR" near Bradford, PA. Better known is the Listowel & Balybunnion of Ireland. India also had a common carrier steam monorail operation, but that equipment used a steel "road" wheel as balance for the monorail.
Hope this helps.
Best Regards,
Glenn