With the completion of the Ryesong River bridge in 1932, the importance of Chōtetsu's Hwanghae Line grew significantly, and by 1935 the company was looking at the possibility - unusual for a narrow-gauge railway - of running limited express trains on the line between Gyeongseong and Haeju. Tests were conducted in 1935 with a Class 630 locomotive, in which it was found that speeds of 70 km/h (43 mph) could safely be reached (the engine's drivers, with a diameter of 940 mm (37 in), reached a rotational speed of 400 rpm).[1]
As a result of the tests, Chōtetsu ordered a 2-6-2 tender locomotive with a driver diameter of 1,100 mm (43 in) to pull express trains at 70 km/h (43 mph) through Hwanghae Province. Designated Class 660, six were built at the end of 1937 by Kisha Seizō (road numbers 660-662, works numbers 1490-1492) and Nippon Sharyō (road numbers 663-665, works numbers 501-503) of Japan.[1]
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