These locomotives were the ultimate development of the Spanish Mastodons. They had a driver diameter of 1,630 mm for mixed traffic and a more powerful boiler with a grate in excess of five square metres and a pressure of 17 bars. Their approved top speed was 105 km/h, but running characteristics were not very good at this speed. Originally designed for the Andaluces, they were ultimately ordered by several railways and built for many years after the RENFE was founded.

The first five were built for the Andaluces in 1935 and numbered 4251 to 4255. Although it was quickly decided to use them as a standard design, the Civil War postponed these plans. Production resumed in 1941, first for the MZA and Oeste and then directly for the RENFE. By 1953, MTM, Euskalduna, Babcock & Wilcox and Devis (later MACOSA) had built a total of 246. The locomotives built from 1950 had double chimneys.

With an output of 2,550 hp, these locomotives were among the most powerful in Europe at this time. They were nicknamed “Renfes” as they had been built for the state railway. On the El Escorial-La Canada line with a ruling grade of 2.1 percent, they hauled fast freight trains of 400 tonnes. Train weight on flatter lines was 850 tonnes. Between 1953 and 1964, all but three were converted to burn oil. Now they were designated 240F.