Atlanta, Birmingham & Atlantic 2-8-2 "Mikado" Locomotives in the USA

Class K-2 (Locobase 7859)

Data from AB&C 11-1930 Locomotive Diagrams and Passenger Car Data book supplied in May 2005 by Allen Stanley from his extensive Rail Data Exchange. See also DeGolyer, Volume 43, pp. 83+ and Volume 50, pp. 67+; and "Mikado Type Locomotives for Atlanta, Birmingham & Atlantic", Railway and Locomotive Engineering, Volume 26, No 3 (March 1913), p. 93. See Works numbers were 38933-38937 in December 1912, 40302-40306 in August 1913, 41739-41743 in October 1914.

These relatively small Mikados came to the AB&C in three batches and were taken into the Atlantic Coast Line as their AK-2. Locobase cannot determine if the 35.7 sq ft (3.32 sq m) of arch tubes is included in the 210 sq ft of firebox heating surface given in both the Baldwin specs and in several AB&C diagram books.

The specs note that four arch tubes will hold the fire brick arch and it was to work "in combination with the Gaines combustion chamber." The combustion chamber consisted of an extension of the firebox that was demarcated by a half-height vertical wall. (Other engines with similar setups are described as having a Gaines wall in the firebox.) Thus, the R&LE report noted, a grate that measured 134" (3.404 m) was actually divided into an 89" (2,261m) long grate and a combustion chamber. For a full description of the Gaines combustion chamber's intended effect on combustion, see Locobase 4228.

Other features included relatively generous 13" (331 mm) piston valves.

Serving the ACL until 1947-1952, many were then sold to short lines and industrial railroads.

Class K-3 (Locobase 13544)

Data from Baldwin Locomotive Works Specification for Engines as digitized by the DeGolyer Library of Southern Methodist University, Volume 34, p. 157. See also "Mikado Locomotive for the Atlanta, Birmingham & Atlantic", Railway Age Gazette, Volume 49, No 7 (12 August 1910), pp. 282-283. Works numbers were 34717-34718 in May 1910.

Surprisingly small grate and firebox on this Mikado, or is the boiler simply huge? That was the question Locobase first posed years before he found the RAG article shown in the first paragraph. That report wondered as well at the disproportion of heating surface to grate:

"The ratio of grate area to heating surface is as 1 to 91.7, which is very much greater than the most liberal practice of a few years ago. in this connection the evaporative work of engines with this large ratio will be of interest and value."

RAG's report referred to the Maintenance of Way Association's Economics of Railway Location Committee's conclusion that a 50:1 ratio produced the highest rate of evaporation. 50:1 owed some of its merit to physical constraints in earlier decades, especially in practical grate area. Still, said RAG, a ratio of nearly double raised the question of "whether the additional steam production corresponds with the added cost and at what point this increase of ratio will cease to be profitable."

RAG also pronounced the relatively low 170 psi boiler pressure was "a matter of interest. It corresponds very closely with Prof. Goss's point of maximum efliciency, and it would be of great value if this matter could be tested to a finish."

In assessing the design, Locobase finds the combination of outside Baker valve gear and slide valves "interesting" as well. Valve travel of 5 5/8" (143 mm) seems short, but the relatively low drivers may have dictated that setting to some extent.

Locobase 7860 shows the 1916 superheated upgrade.

Class K-3 - superheated (Locobase 7860)

Data from AB&C 6-1928, 6-1942, and 1-1943 Locomotive Diagrams and Passenger Car Data book supplied in May 2005 by Allen Stanley from his extensive Rail Data Exchange.

Locobase finds it very hard to believe that superheating the Mikados shown in Locobase 13544 would also result in a grate area as small as 43.3 sq ft, yet all of the AB&C diagram books insisted on that figure. This represented a 15.2 sq ft decrease from the original grate in the bloated saturated boilers originally delivered with these engines in 1910. In the same makover, 210 of the 2 1/4" tubes were removed in favor of 40 flues.

(NB: All of the diagram books prior to 1943 show much higher evaporative heating surface areas. Locobase believes that the figure given included the superheater area, which is also shown separately. The math was corrected in the 1943 edition, which is the source of Locobase's specifications.)

The diagrams suggest that a combustion chamber in the boiler, but there's no clear indication of such. Also the firebox had four arch tubes, which would have added about 28 sq ft (2.60 sq m) to the direct heating surface. But the ratios were still very high and the factor of adhesion quite low, despite the addition of 10,000 lb (4,535 kg) of "dead weight" to the drivers in October 1939 (3020 and April 1940 (301).

The diagrams note that a May 1925 shop stop saw the installation of a feedwater heater and a mechanical lubricator in 302; 301 received a similar upgrade in December. The 1929 makeover added a "Hylastic" steel frame, new piston valves and cylinders, ACL standard binders, and improved Baker valve gear. Automatic stokers were added in 1940.

After the ACL took over the AB & C in December 1945, these two were redesignated AK-3. 301 suffered a wreck in June 1946 and was scrapped. 302 went to the scrapper in April 1947.