Some define Tinplate trains as Toy trains. "For serious railroad aficionados, the word “tinplate” is synonymous with “toy trains,” meaning a miniature train that is not made to scale. Toy trains often have oversize elements like smokestacks or wheels. Model trains, by contrast, are authentic replicas of real trains, correctly proportioned right down to the smallest handrail." from http://www.collectorsweekly.com/model-trains/tin .
Another "TIN-PLATE - A term coined to refer to the original (and classic) American Flyer, or Lionel trains. Usually these toy trains were constructed with certain liberties taken in the size and dimension of the models to allow for ease of construction, or to keep the overall dimensions of the train layout to a size that would fit in a house. The term tin-plate was chosen because the early trains were usually constructed of stamped and printed tin. Later, of course, plastics and other materials were used." from http://www.trainweb.org/sjsg/definition.htm.
These definitions seam to apply to "3 rail O gauge trains term" also. O gauge implies not to scale and O scale means complying to 1/4 inch per foot or 1:43 or !:48 scale (lets not get into this here).
So what is the difference in the 3 rail O gauge train forum and the Tinplate train forum focus?
Charlie