Articulated locomotive: Difference between revisions

An '''[[articulated vehicle|articulated]] locomotive''' is a [[steam locomotive]] (rarely, an [[electric locomotive]]) with one or more engine units that can move independentindependently of the main frame. Articulation allows the operation of locomotives that would otherwise be too large to negotiate a railroad's curves, whether mainlines or special lines with extreme curvature such as [[LoggingForest railroadrailway|logging]], [[Industrial railway|industrial]], or [[mountain railway]]s.

Articulated locomotives saw service in many nations, but were very popular on [[narrow-gauge railways]] in Europe. The largest examples were developed in the United States, where the [[Union Pacific Big Boy]] [[4-8-8-4]]s and the Allegheny H-8 [[2-6-6-6]]s were some of the largest steam locomotives ever built, with [[Big Boy 4014]] remaining as the largest, and last of its kind to still operate.

Many schemes for articulation were developed over the years. Of these, the [[Mallet locomotive]] and its simple-expansion derivative were the most popular, followed by the [[Garratt locomotive|Garratt]] type (mostly built in the [[United Kingdom]], popular throughout [[Europe]], [[Africa]] and European colonies), and the various [[geared steam locomotive]] types, the latter largely used in logging, mining and industry. Most other types saw only limited success.

* The [[Fairlie locomotive|Fairlie]], with two powered trucks under a double [[locomotive boiler|boiler]], or its [[Fairlie locomotive#Single Fairlie locomotive|Single Fairlie]] single-boiler derivative with one powered and one unpowered truck (known as a [[Mason Bogie]] in the [[United States]]).

* The [[Garratt locomotive]], with an engine unit at each end carrying coal and water supplies, and a boiler unit articulated between them.

[[Simple expansion steam engine|Simple expansion]], or simple, articulated steam engines had two sets of equally sized cylinders. High-pressure steam was supplied to all cylinders and exhausted out of the stack once it had been used. The American simple-expansion articulated, thanks largely to the smaller mass of the forward cylinders when compared to the compound-expansion Mallets allowing for higher piston speed, were generally better suited for high speed than their compound cousins. Examples of the "simple mallet" design include the [[Union Pacific Big BoysBoy]]s and [[Union Pacific Challenger|Challengers]], B&Othe [[Chesapeake and Ohio class EMH-1s8]], the [[2-8-8-4]] "Yellowstones", the majority of [[Southern Pacific Transportation Company|Southern Pacific]]'s [[Cab-Forwardsforward]]s, and the N[[Norfolk &W Western]] [[Norfolk and Western Railway class A|A-class]].

[[Compound expansion steam enginelocomotive|Compound expansion]], or compound, articulated steam engines like [[Anatole Mallet]]'s original idea, consist of two sets of unequally sized cylinders. The smaller pair of cylinders near the cab was fed with high pressure steam directly from the boiler and then the steam was passed into a pair of low-pressure cylinders at the front, with larger diameter to offset the lower pressure, before exhausting through the smokestack. While the [[thermal efficiency]] was greatly improved through the compound use of steam in Mallet designs, the large low-pressure cylinders posed unique limitations, both in terms of loading gauge (the cylinders could only be as large as the track and track-side infrastructure allowed) and in terms of performance at speed. The large and consequently heavier pistons caused stability issues at higher speed, which generally limited compound expansion articulated locomotives to below 30 or 40 miles per hour. A notable exception to this was to be found in later iterations of Norfolk & Western Y-class 2-8-8-2s, which could and did often exceed 50 miles per hour in service as well as being one of the hardest-pulling steam locomotives ever built.

The first Garratt locomotives constructed, the [[Tasmanian Government Railways K class]] were also compound locomotives, but were complicated as a result. All subsequent GarratsGarratts were simple engines only.

** [[FS Class E656E.636]] operating in [[Italy]], first built in 1940

** [[New Zealand [[NZR EW class locomotive|New Zealand EW class]], built by [[English Electric]] in 1952.

* Wiener, Lionel, ''Articulated Locomotives'', 1930, reprinted 1970 by [[Kalmbach Media|Kalmbach Publishing Company]] as {{ISBN|0-89024-019-1}}