Buenos Aires Pacific Tipo 20

The Tipo 20 comprised 14 Atlantic locomotives built by North British in 1905 and numbered 600 to 613. Although their bulky, compact appearance suggested otherwise, they were in fact relatively large locomotives, weighing 75 tons and having driving wheels measuring 6 ft 8½ in (2,045 mm) in diameter. They featured a Belpaire firebox with a grate area of 30 square feet (2.8 m²), outside cylinders, and inside Stephenson valve gear.

After only seven years of service, the first members of the class were rebuilt as Pacifics at the Junín Workshops. This conversion involved lengthening the frames and adding a third coupled axle. At the same time, the boilers were raised and larger cabs were installed.