The articulated 2-6-6-2 of the class HR-01 were the heaviest locomotives used by the Mexican State Railway on their three-foot narrow gauge network. In general dimensions they were similar to the Uintah Railway No. 50 and 51, but were tender locomotives. Without the need to store water and fuel oil on the locomotive itself, they had a larger boiler. The included a wide firebox with 55 square feet of thermic syphons. The drivers had a diameter of 43 inches and the cylinders measured 15 by 22 inches, controlled by Baker valve gear.
The first six were built by ALCO-Schenectady in 1928 with the works numbers 67707 to 67712. They were initially numbered 240 to 245, but renumbered 361 to 366 in 1930. Numbers 367 and 368 were built in 1934 with the works numbers 68644 and 68645. These were the only ones with an Elesco feed water heater. The final two, numbered 369 and 370, were delivered in 1936 with the works numbers 68773 and 68774. The four locomotives built in the thirties were six tons heavier and had a larger tender that contained 3,000 instead of 2,500 gallons of oil.
They were used in passenger and freight service. On the level they were rated for a train of 800 tons at 60 km/h or 37 mph. On a two percent grade they managed to reach 25 km/h (16 mph) with 410 tons and on four percent, it was 12 km/h (7 mph) with 230 tons. Their tasks became less with the conversion of narrow gauge lines to standard gauge. One was taken out of service in 1951 and the others in 1953.
