July 18, 2004
As America’s railroads grew over the years, trains got longer and locomotives got bigger. The longer a train, the more productivity they could get out of each crew. In 1904, the B&O found a way to get twice the power in a single locomotive, by combining two engine mechanisms under the same boiler. This new type of locomotive was called a Mallet. It revolutionized mountain railroading by pulling longer trains up the hills with far lower crew costs. Before we get confused with terms, let’s cover a few definitions:
“Articulated” means two or more sets of driving wheels, one of which can hinge on curves.
“Compound” means a system that uses steam twice, with one set of pistons larger than the other.
“Mallet” means a compound articulated locomotive.
“Simple articulated” means an articulated locomotive that sends the same steam pressure to all cylinders.
The first successful Mallet in the U.S. was B&O’s 0-6-6-0 of 1904. Its success drove many railroads to order Mallets for pulling longer and longer trains. This was the “drag freight” era for railroads, where speed was not as important as train length, and the Mallet fit in nicely. Several ideas were tried to increase power, efficiency, and speed. The Erie tried a 2-8-8-8-2 and the Santa Fe tried flexible boilers. Neither idea worked. In 1919 the Virginian ordered a set of 2-10-10-2's that was very successful, and had the highest starting tractive force of any steam locomotive ever, 176,600 pounds. One of the advantages of compounding was that if one engine set slipped, the other got extra steam, balancing the power output. Even with large and efficient Mallets, the railroads found a few drawbacks: 1) back pressure caused the front cylinders to lose efficiency over 15-20 mph, 2) the large cylinders and small wheels limited counterweights to balance wheels at lower speeds, and 3) the huge front cylinders meant the railroads needed larger tunnel clearances. Therefore the Mallet was best at pulling and pushing long trains at low speeds where clearances allowed. The Norfolk and Western had plenty of need for high-power low-speed locomotives, so they continued to develop their Y-class Mallets into successively better locomotives right up to the late 1950's with the Y-6b. An earlier version, the Y-3, was chosen as the basis for Aristocraft’s new Mallet locomotive.
Simple Articulateds
In 1919 the Pennsylvania Railroad designed a large Simple Articulated that was ahead of its time: when it was used at the front of a train, it broke the couplers far too often. A few years later, in 1924, a tunnel-clearance problem convinced the C&O to try a Simple Articulated with the same basic size of a Mallet but with smaller front cylinders. The concept worked well. Then the Great Northern built a Simple Articulated with larger drivers that could run at much higher speeds. Now the true value of a Simple Articulated was recognized: high power AND faster speed. Several railroads, including the Reading and B&O, converted their existing Mallets into Simple Articulateds. Others developed new designs that were larger and faster. The Southern Pacific had a problem with smoke and fumes choking engine crews in tunnels. It solved it by building Simple Articulateds with the cab in the front (Cab Forwards). The quest for high power and high speed culminated in the Union Pacific’s Big Boy, a huge 4-8-8-4 that could run 70 mph with heavy trains.
In the 1940's the Pennsylvania Railroad experimented with the idea of building engines with four pistons instead of two, so that the reciprocating machinery could be lighter and thus capable of higher speed. They were huge and powerful and looked like articulated engines, but actually had a rigid wheelbase. The freight version was the Q-2 4-4-6-4, the longest and most powerful steam locomotive in the world. The passenger version was a 4-4-4-4, known as the T-1, a handsome and fast engine, easily capable of 100 mph. They were not considered a successful design because of maintenance problems; and were all retired within 6 years.
Steam Turbine Locomotives
Several coal-hauling railroads attempted to develop new types of locomotives to use coal as fuel. The Pennsy tried with a locomotive that had a turbine mounted directly to the running gear. The Norfolk and Western tried with the “Jawn Henry,” in which the steam turbine powered a generator which sent power to traction motors at the wheels. The C&O designed a fleet of three similar steam-turbine-electrics. None of these designs worked well enough to compete against the diesel-electric locomotives that were already replacing steam.
Union Pacific’s “Big Boy” is often called the largest locomotive ever built, but even if it wasn’t technically the biggest, it was certainly one of the best. Several are on display throughout the country, including one at Steamtown, but none are in running condition.
Union Pacific’s “Challenger” 4-6-6-4 was just as fast, although not quite as large or powerful. Challenger #3985 is still running specials for the U.P., and is based in Laramie, Wyoming.
Norfolk and Western’s Class A 2-6-6-4 was a fast, powerful, efficient locomotive used for both freight and passenger trains. Number 1218 ran steam specials until a few years ago. The Norfolk and Western’s Y-class and A-class locomotives were featured in many fine photographs taken by O. Winston Link in the 1955-1960 period.
The Southern Pacific owned more articulateds than anyone else, 207, including 195 Cab Forwards.
Chesapeake and Ohio’s tough-looking “Allegheny” 2-6-6-6 was, by most measurements, the heaviest and most powerful steam locomotive ever built. One is on display at the B&O Museum in Baltimore.
1:22.5 LGB Uintah “Mallet” 2-6-6-2T #50 and 51. Prototype 3 ft gauge, technically a Simple Articulated.
1:22.5 LGB Sumpter Valley “Mallet” 2-6-6-2. Prototype: railroad-modified Uintah engines..
1:32 MTH Challenger 4-6-6-4. Prototype: Union Pacific Challenger.
1:32 Accucraft Cab Forward. Prototype: Southern Pacific #4274, 4294.
1:29 Aristocraft 2-8-8-2 Mallet. Prototype: Norfolk and Western Y-3 Class.
Books:
"The Iron Horse" by Henry B. Comstock
"Reading Power Pictorial" by Bert Pennypacker
Magazine articles from Trains Magazine:
Jan 2002: "Simple Articulateds" by Neil L. Carlson
June 2000: "Super-Power from Berkshire to Big Boy" by Neil L. Carlson
April 2004: "Big Locomotives for Big Grades" by J. David Ingles
April 2004: "Getting Them up the Grade the Norfolk and Western Way" by Ed Kng
Internet: www.steamlocomotive.com/misc/largest.html