The first batch of fifteen R.301s, built by Costruzioni Meccaniche di Saronno (CEMSA), was delivered to the FS in 1912.

[1] In the following years, the fleet was enlarged by an additional eightteen units whose construction was split between Società Italiana Ernesto Breda of Milan, Gio.

[2][1] The R.301s were designed to solve the operational problems of the preceding locomotive designs, above all the poor performance of four axle, rigid wheelbase R.401s which damaged the track in the numerous curves typical for narrow gauge alignments.

Upon testing, the R.301s exceeded expectations; on a constant slope of 25‰ (2.5%, 1-in-40) and curves with a radius of 100 m (330 ft), they were able to haul 90 t (89 long tons; 99 short tons) at 25 km/h (16 mph) The boiler of the R.301 had a 74.5 m2 (802 sq ft) heating surface and the cylinders had a bore of 380 mm (14+15⁄16 in), a stroke of 450 mm (17+11⁄16 in), which gave a starting tractive effort of 6,750 kgf (66.2 kN; 14,900 lbf).

This highly reliable system creates vacuum in the pipe by the use of a steam ejector and has no moving parts.