The SAL operated a total of ten 2-6-6-4 articulated locomotives which were built in two batches of five by Baldwin in 1935 and 1937. Although they were smaller than the Norfolk & Western class A, they still weighed 480,000 pounds without a tender and 790,200 pounds in total. The grate measured 96.3 square feet (8.95 m²) and the firebox had a surface of 515 square feet (47.85 m²), including two thermic syphons and a long combustion chamber. The tubes and flues totaled 4,998 square feet (464 m²) and the superheater 2,397 square feet (223 m²).

The first batch was designated R-1 and numbered 2500 to 2504. They featured Baker valve gear and an Elesco feed water heater. The second batch was called R-2 and carried the numbers 2505 to 2509. They were slightly modified with Walschaerts valve gear, a Worthington feed water heater and only 38 instead of 44 tubes. Like many articulated locomotives with three sets of drivers per group, they tended to slip on the front one.

All ten were bought by the Baltimore & Ohio in August 1947. The R-1 was now designated class KB-1 and numbered 7700 to 7704. The R-2 became class KB-1a and was numbered 7705 to 7709. Apparently the B&O had to fix the flexible steam joints on the front engine since they leaked frequently. In general, they were not as popular as the class S-1 2-10-2 locomotives since these were less likely to slip and had almost exactly the same starting tractive effort. All ten were scrapped in May and June 1953.