An articulated locomotive is a steam locomotive (rarely, an electric locomotive) with one or more engine units that can move independent 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 logging, industrial, or mountain railways. 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 4-8-8-4s and the Allegheny H-8 2-6-6-6s were some of the largest steam locomotives ever built. 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 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. As distinct from articulated locomotives, a non-articulated locomotive is referred to as a straight or rigid locomotive. Articulated steam locomotive types 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, 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