The Seaboard Air Line operated ten 2-6-6-4 articulated locomotives built by Baldwin in two batches of five in 1935 and 1937. Although smaller than the Norfolk & Western class A, they were still substantial locomotives, weighing 480,000 pounds without tender and 790,200 pounds in working order. The boiler featured a 96.3-square-foot grate and a 515-square-foot firebox equipped with two thermic syphons and a long combustion chamber. Heating surface amounted to 4,998 square feet in the tubes and flues and 2,397 square feet in the superheater. 

The first batch, class R-1 (Nos. 2500–2504), was fitted with Baker valve gear and an Elesco feedwater heater. The second batch, class R-2 (Nos. 2505–2509), differed in having Walschaerts valve gear, a Worthington feedwater heater, and 38 rather than 44 boiler tubes. Like many articulated locomotives with three coupled axles per engine unit, they were prone to slipping on the leading engine.

All ten locomotives were sold to the Baltimore & Ohio in August 1947. The R-1 locomotives became class KB-1, numbered 7700–7704, while the R-2 locomotives were classified KB-1a and renumbered 7705–7709. The B&O reportedly had to address persistent leakage problems in the flexible steam joints supplying the front engine unit. In service they proved less popular than the S-1 2-10-2s, which developed almost identical starting tractive effort while being less susceptible to wheel slip. The entire class was withdrawn and scrapped in May and June 1953.