Farouk--
Welcome to the GE locomotive forum!
The majority of the participants on this forum are based in North America, and most -- BUT NOT ALL -- of the locomotives discussed are from GE's "domestic" line: U14/15 and U6/8 are "export" models (though a very small number of U6 were built for a North American railroad). From a historical point of view, it seems to me unfortunate that so many Americans interested in locomotives ignore the "export" line: they provide an important context to the evolution of GE's "domestic" diesels.
U-6 and U-8 are, as you suggest, "Switchers," in the sense that they have end-cab carbodies. I think the U-6 designation was first used for a design powered, like GE's domestic "70-tonner," by a six-cylinder Cooper-Bessemer diesel engine: an inline diesel design which, in V-format, evolved into GE's FDL engine. (GE ultimately bought the engine design from C-B; FDL engines since the early 1960s have been built by GE: there was a long discussion on this forum of the history of the FDL engine design some time ago which has been saved at the top of the topic index.) It was later given (GE has several times reapplied model designations to distinct designs) to a GE locomotive with a Caterpillar diesel engine (the few U-6 in North American service were bought by a short line which already had a fleet of 70-tonners, and took the Caterpillar-engined units as a substitute when they wanted more after the end of 70-tonner production); uprated Caterpillar engines in similar locomotives gave the U-8 and U-10 models.
U-12, U-13 and U-15 would have been "road-switchers" (= carbody with a short hood "nose" in front of the operator's cab) with versions of the 8-cylinder FDL engine: U-12 would have been the first, introduced when GE announced the "U-series" ("Universal Series") of locomotive designs in 1956, U-15 for versions with the engine uprated to a rating reached in the mid-1960s. Ultimately a developed version of the same V-8 engine was used in a "U-20" model: "U-20" having been used for a locomotive with a 12-cylinder FDL engine in the 1960s.
There is a very nice article on the evolution of GE's "export" U-series locomotive designs, by Steve Palmano, at
http://www.locopage.net/ge-history.htm
(There is also an article by Steve Palmano on some earlier GE export locomotive designs in the "files" section of the "World Diesel Locomotives" Yahoo group.)
Do you have "personal acquaintance" with some of these locomotives? If so, where? And can you tell us more about them?
