TT-Gauge is smaller than the more common HO-scale size, but larger than Z & N-Scale. TT-Gauge is considered to have been established by Mr. Hal Joyce (Indiana, USA). Mr. Joyce's company, H.P. Products, produced TT gauge trains from about 1946 until 1965. Rokal's production sort of followed this pattern, starting manufacturing model trains in about 1947 and ceasing in late 1969 upon the death of Robert Karmann.
Rokal was originally founded in Dusseldorf in 1914 as a metal casting company. The company later moved to Lobberich in 1926, becoming Rokal. Rokal is an acronym for RObert KAhrmann Lobberich, a manufacturer of metal castings that produced a line of model railway equipment in TT scale from 1947 - 1969, based in Lobberich 80 km north of Köln (Cologne), Germany at the Dutch border. The Rokal product line was then sold to Willy Ade of Röwa. Röwa issued at least one catalog that included items from the Rokal product line, but apparently only sold existing inventory from the Rokal purchase rather than manufacturing these items. Ade later sold his company to Roco of Salzburg, Austria and the Rokal TT line was not issued again.
Rokal began manufacturing its model train line in the late 1940's, probably around 1948. Initial model train development was under the direction of Eugen Englehard, who left the company in the very late 1940's. Development continued under the direction of Heinz Thieme.
At that time, TT-Scale was in its infancy, yet to be discovered by model railroad enthusiasts around the world. Even the Rokal company itself was a bit reluctant about its role in the model train world. The words describing the Rokal factory in the Company's catalogs always were just a bit evasive. 'Covering an area of almost 24 acres, ROKAL employs over 2,000, in extensive and modern facilities, in which ROKAL TT gauge model trains are produced as a part of widely diversified metal products'. This is technically accurate, but the key phrase here is '...part of widely diversified metal products', since the Rokal factory also produced plumbing fixtures, automobile carburetors, and architectural hardware such as door handles. In short, metal castings.
The electric train business was a sideline for Rokal, more the hobby of the owner, Robert Karmann. Yet, the nature of this manufacture did not detract from the quality of their electric train products. The existence of a profitable manufacturing facility insured that Rokal trains would be produced to high standards of quality materials.
The Rokal product line was of slightly different proportions from other TT scale lines. In later years of production, Rokal worked in joint venture with firms located in the former country of East Germany. The products of these companies were offered by Rokal with the Rokal style coupler and offered in their own product line with their proprietary coupler. These companies apparently used the same set of manufacturing tooling, marketing these items under names such as Zeuke, Zeuke & Wegworth, Berliner TT-Bahnen (in short, BTTB) and possibly others.
Source and more information: guidetozscale.com
Other websites: Rokalfreunde Lobberich
What's it worth? Take a look at this Rokal price guide: sold listings for a value indication.