Class ML-55 (Locobase 10031)

Data from William Ernest Dalby, Steam Power (London: Longmans, Green, and Co., 1915), p. 269, supplemented by Baldwin Locomotive Works Specification for Engines as digitized by the DeGolyer Library of Southern Methodist University, Volume 40, pp. 138-149 and from "MoPac 2-8-8-2 No. 4000 "Molly" the Mallet -The Mightiest of them All," T Greuter ([]). (Thanks to Chris Hohl for his 6 March 2018 email noting Chris Ecoff's "Molly the Mallet: Mopac's Extreme Steam Machine", Classic Trains, Volume 14, No 2 (Summer 2013), pp. 86-89 and its information on the Bethlehem tender boosters.) Works number was 37675

in April 1912.

A pretty large fish out of water, this single Mallet's boiler included 1,230 sq ft (114.27 sq m) of feedwater heater. When 4000 arrived on the railroad, it almost immediately acquired a nickname - Molly - which reflected both the tendency of engine crews to think of their locomotives as iron women and a probable play on the usual pronunciation of Mallet ("Malley").

Molly ran on the StL & IM and the Missouri Pacific as a hump switcher until 1946. T. Greuter, writing on the Trainweb website, offered an affectionate description of Molly's career. First he tells us what she didn't do: "It is easy to imagine that such an impressive engine as Molly may have been put to use on the most powerful runs of the day, cresting the limestone rises of Kirkwood with a mile-long line of coal freight, but she didn't."

So, a grand vision, yes, but not what she did, as Greuter explains:

"The fact is Molly was a blue-collar worker, through and through. Her sheer massiveness made her ideally suited for effortlessly shuffling the high tonnage in endless strings of cars, breaking-up and building trains at Missouri Pacific's yard hump in St. Louis and Dupo, as well as tending transfer drags ... a sight that must have truely been awe inspiring to witness."

And, he notes, "Nothing else owned by the road approached her, Molly was the single one of her kind on the system."

Hohl, citing Ecoff, noted that Molly and one of the Missouri Pacific's 2-10-2s were modified in the Sedalia shops in 1927. Among the changes, the most significant was the installation of Bethlehem tender truck booster engines on both trucks. The Bethlehem auxiliary engine was not unique to Molly, others having been installed in Lehigh & New England 2-10-0s (Locobases 65 and 14277) and 2-8-0s (Locobase 412), Lake Superior & Ishpeming 2-8-0s (Locobase 3295), Alton & Southern 0-10-0s (Locobase 13993) and 2-8-2 (Locobase 8318), Union Railroad 0-10-2 (Locobase 5976), and one of the L&N J-1A class of 2-8-2 (Locobase 8124), Pittsburgh & West Virginia's first three 2-6-6-4s (Locobase 301), and Lehigh Valley's T-1a 4-8-4s (Locobase 263).

Installing boosters on both trucks was rare; only the Norfolk & Western Y-3 2-8-8-2 road number 2006 (Locobase 1421) and Bessemer & Lake Erie 2-10-2 #505 (Locobase 7655) also drove both trucks.

Hohl notes that the new tenders carried 15 tons of coal & 18,000 gallons (68,130 litres) of water. Ecoff's account said that the design included "automatic stokers and dual 10-inch diameter exhaust pipes for the boosters atop each end of the water space". In each three axle truck, two of the axles were connected by side rods.

The two boosters provided an additional 30,000 lbs(13,608 kg/133.45 kN) in starting tractive effort for a estimated total of 124,000 lbs.). Ecoff added that in 1937, Molly's massive and headache producing tender was replaced by the tender from 4-6-2 # 6623

This tender rode on Commonwealth passenger trucks, used a new company-built tank, carried 16 tons of coa delivered by automatic stokerl & 10,000 gallons of water.