Major types The major types of articulated locomotive are: • The
Fairlie, with two powered trucks under a double
boiler, or its
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. • The
Mallet locomotive, with one fixed engine under the rear of the locomotive and a radially swinging engine unit in front. • The
Meyer locomotive, with two powered engine trucks under the locomotive (generally with the cylinders inward). Also, the Kitson-Meyer variant with the trucks spread apart to allow a deeper
firebox between them.
Simple expansion 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 Boys and
Challengers, the
Chesapeake and Ohio class H-8, the
2-8-8-4 "Yellowstones", the majority of
Southern Pacific's
Cab-forwards, and the
Norfolk & Western A-class.
Compound expansion 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 Garratts were simple engines only.
Geared types There were various types of articulated
geared steam locomotive, including: •
Climax locomotive •
Heisler locomotive •
Shay locomotive •
Willamette locomotive Other types •
du Bousquet locomotive •
Engerth locomotive •
Gölsdorf axle •
Golwé locomotive •
Hagans locomotive, such as the
Prussian T 13 (Hagans variant) •
Klien-Lindner locomotive •
Krauss-Helmholtz bogie •
Luttermöller axle ==Electric locomotives==