TT scale is a niche model railroading scale, whose name stands for Table Top. TT gauge was the invention of an engineer by the name of Hal Joyce around 1940, who also created HP Products, but commercial production was delayed until 1945 because of the war. HP brought out an extensive line of locomotives, cars, track, and accessories. Other manufacturers also joined the ranks of TT scale producing items. Gandy Dancer, Jewel, Craftsman, Star-Line, Kemtron, and Christoph Products just to name a few. TT was a fairly popular scale through the 1950's.
TT scale is a model railroad scale whose size falls in between HO and N gauge. It's actual scale measurement is 1/120, or 0.10 inch = one foot, or one inch = ten feet. It's track gauge was set at 0.471 inch or 15/32. 12mm is currently the accepted gauge, which is 0.472 inch. TT scale was once the smallest practical model railroad scale available, and quite popular, even though it was (and is) primarily a craftsman's scale.
TT retains a comparatively small niche in the United States and in the United Kingdom, but growing popularity in the former East Germany; it is the most popular scale in Eastern Europe and Russia. Adherents to TT maintain it is the smallest practical scale, especially for those who like to build models from scratch.
By the early 1960s, TT had been eclipsed in popularity by N scale, which is smaller. TT scale manufacturers were Rokal and Zeuke, which later became Berliner TT-Bahnen and ultimately Tillig.
What's it worth? Take a look at this Tt price guide: sold listings for a value indication.