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Example of an articulated 2-8-8-2 Mallet class locomotive

MoPac 2-8-8-2 No. 4000 "Molly" the Mallet -
The Mightiest of them All
by T. Greuter

Born at the close of the Steam Era, Molly was not the largest Mallet ever built, but don't be fooled. The Mallet engine such as the Baldwin-built 2-8-8-2 MoPac #4000 was a wonderously huge example of sheer brute force... these were the most powerful steam locomotives to be seen in the U.S. Imagine a very long and powerful engine with two full sets of drive wheels, the front set being articulated from the body to allow the engine to ride a curve. Like all Mallets, Molly was articulated, meaning she had at least two sets of drivers with one able to swing away from the boiler... making for an enormous traction base. When taking a tight curve the boiler could overhang the rails considerably. The Mallet locomotives also reused steam from one set of cylinders to another. 

Anatole Mallet and his Grand Design
Mallets were commonly used by the railroads for slow but steady transport of lengthy coal drags, and many (especially the earlier versions) found use in the logging industry. Locomotive builder Baldwin didn't manufacturer the first Mallet, but they excelled like no other. The Baldwin-Mallets were mainly 2-6-6-2's, 2-4-4-2's and 2-8-8-2's, though similar arrangements used were 4-8-8-4's which arguably reached somewhat past the limit that steam technology could handle. Even bigger arrangements were proposed in Europe but never made a reality, such as the 0-6-6-0+0-6-6-0 Garratt-Mallet.

The name "Mallet" comes from the designer of articulated compound locomotives - Anatole Mallet (pronounced "Malley"). To be a true Mallet locomotive, the engine must be articulated and be a compound, meaning the steam is used twice in separate cylinders. A simple explanation of compounding follows:

The high pressure cylinders use the steam first and are the smaller of the two sets, the steam is then passed to the larger low pressure cylinders where the steam is used again. It was supposed to be more efficient, but the complication was maintenance intensive, and the locomotives were actually slower than simple expansion locomotives.

The creation of the first Mallet was simply one solution for the never-ending quest for more horsepower. To achieve more horsepower an engine requires more heating surface and grate area. More heating surface means larger boilers. A larger boiler means a longer, heavier locomotive with more boiler capacity and the greater tractive effort exerted by a larger number of drivers. There was one real problem in order for this idea to work on rails... the fact the mega-engine would need to round curves. So the articulation principal of Anatole Mallet's was very appealing to the buyers of the first Mallet, as well as the improved thermal efficiency from Mallet's approach to compounding.

The first Mallet locomotives (early 1900's) were designed to meet the needs in Europe. The US Mallets which came later required a number of innovations to deal with an engine that is typically twice the size of its European counterpart.  Originally these engines were used as pusher engines on grades as steep as 2.5%.   In addition to solving traffic congestion the Mallets were very fuel efficient, reducing the amount of fuel used per ton mile by 46%. (for more Mallet info see

MoP's Mallet "Molly"
Affectionately dubbed with the name "Molly" by her crews... she was delivered new in 1912 to the St. Louis Iron Mountain & Southern, a predessesor to the Missouri Pacific and pressed into service in the Dupo and St. Louis area, where she tended transfer drags and hump duties. This is where she spent her entire career, never to stray far from her original assignment.

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

Other than her size, Molly was your typical MoPac steam engine with all the standard features. In our earliest photo, on the MPHS website ( we see Molly as she looked prior to the merger of the MP with the Iron Mountain. Under her cab window in small capital letters is the name "IRON MOUNTAIN", on her middle dome is the number "4000". Her boiler is a shiney black with her smokebox a flatter graphite color. Notice she had smaller domes and oil-powered headlamp arrangement.

Photographic evidence shows that as time went by, beyond the merger into Mopac, she recieved a number of modifications. Besides regular shopping and rebuilding dictated by the railroad, steam crews were consumate tinkerers and constantly tweaked and made improvements to their charges, much more so than their diesel counterparts. All steam engines became unique and one of a kind pieces of machinery.

Most noticeble of Molly's changes are the larger forward and rear domes, additional "plumbing" mounted along her flanks, a spark arrester/flap for her smokestack, and a more modern electric headlamp. Her tender seems to be the original car, which also recieved new trucks and a "doghouse" for the head brakeman.

In a 1936 photo (MPHS Eagle, Vol. 19 No. 1) she's seen painted in the conservative Missouri Pacific colors - an all-over glossy black boiler and a brighter graphite, almost aluminum on the smokebox and firebox. A large brushed aluminum "4000" was applied under her cab window with "M.P." sublettering. The sublettering was also placed on either side of her squat forward dome.

The Wallin Collection photo, also on the MPHS website ( apparently shows the locomotive at a later date, covered in sooty grime - both the smokebox and boiler weathered to the same shade.

MO&G 303
- Missouri Oklahoma & Gulf is a Baldwin 2-6-6-2 Mallet in Muskogee, probably on its delivery. The contributor's granddad is at front of engine in white shirt and overalls. On the back of the engine picture:

"Doc Epperson in white shirt taken in Muskogee Oklahoma 1911 or 13 QO&KC railroad."

Markings on engine and tender indicate it was #303 of MO&G. - Brice Bratcher Photo/Collection

Note on MO&G / KO&G 2-6-6-2 Mallets
On a related note, Texas Pacific/Missouri Pacific subsidiary road Kansas, Oklahoma & Gulf Railway inherited five Baldwin-built 2-6-6-2s, #300-304. These five originally had been built for KO&G predecessor Missouri Oklahoma & Gulf in 1912.
KO&G replaced these 300-series units with Sante Fe's after 15 years of service.



Wheel Arrangement: 2-8-8-2

Class: ML-55

Tank or tender type: Rectangular tender

Builder / date: Baldwin - 1912

Serial number:

Driver diameter: 55"

Boiler pressure: 200 PSI

Cylinder dimensions: 26x32 LP / 40x32 HP

Grate area:

Tractive effort:  94,400

Weight: 454,400


Water capacity:

Service History: 1912 to St. Louis Iron Mountain & Southern as #4000 Missouri Pacific RR. as #4000. Assigned to St. Louis and Dupo hump duties entire career.

Disposition after MPRR service: Sold to...? Scrapped... ?

Life-Like’s Proto 2000 Heritage 2-8-8-2 Heavy Mallet in HO Scale
Rivarossi Steam 2-8-8-2 Mallet in HO Scale
Also occasionally seen in Brass

Pictures: MPHS website - im_4000.jpg mp4000.jpg mp_4000.jpg

Mallet Baldwin 1912 1912 1 unit


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 l Last Update to this page: 29 May, 2008
          All images & text © 2000-2008 T. Greuter / Screaming Eagles, unless otherwise noted. All Rights Reserved.