Upon introduction in 1956, nine locomotive models were offered, as follows: The evolution of Universal Series locomotives is complex due to the frequent model name changes and the reusing of the same model names for different locomotives. One method to understand the growth and development of the Universal Series is to divide the nine original models into three groups: 1) small locomotives, 2) intermediate powered locomotives, and 3) high powered locomotives.
Small Locomotives In this table, and the following two tables, asterisks indicate repeated model numbers Between the initial models and the first update, the Caterpillar 375 and 397 engines were replaced with 379 and 398, respectively. The UM6B was a narrow-gauge variant of the U6B, built for South African Railways. The 6-cylinder U9B and U9C models were an evolutionary dead end, and no uprated versions were produced.
Intermediate Powered Locomotives (8-cylinder) Between the first and second updates, the Cooper-Bessemer engine was replaced with 7FDL8 engine. The body style was also changed with a low short hood and Hi-Ad trucks replacing the high short hood and outside-equalized trucks. A variation of the U18C was the U18A1A, built for the
Indonesian railway. These locomotives had the same C-C trucks but one fewer traction motor in each truck, and downrated to 1650 horsepower. Most of these locomotives have been rebuilt to U18C standards.
High Powered Locomotives (12-cylinder) As with the intermediate powered locomotives, the Cooper-Bessemer engine was also replaced with 7FDL12 engine. The differently-rated U22C, U26C and U30C models were offered at the same time. The U18C and the earlier U20C model had a variant with an additional non-powered leading axle in each truck, resulting in a lower axle loading and a 1C-C1 wheel arrangement. These locomotives were only purchased by the
South African Railways. No U18B was produced, and the few UD18 locomotives built were exported to
Mexico. == Universal Series statistics ==