A class of engines known as "Moguls," with three pairs of drivers connected and a swing pony-truck in front equalized with the front drivers, took its rise in the practice of this establishment from the "E. A. Douglas," built for the Thomas Iron Company in 1867. These engines are fully illustrated in the Catalogue. Several sizes of "Moguls" have been built, but principally with cylinders sixteen, seventeen, and eighteen inches in diameter, respectively, and twenty-two or twenty-four inches stroke, and with drivers from forty-four to fifty-seven inches in diameter. This plan of engine has rapidly grown in favor for freight service on heavy grades or where maximum loads are to be moved, and has been adopted by several leading lines. Utilizing, as it does, nearly the entire weight of the engine for adhesion, the main and back pairs of drivers being equalized together, as also the front drivers and the pony-wheels, and the construction of the engine with swing-truck and one pair of drivers without flanges allowing it to pass short curves without difficulty, the "Mogul" is generally accepted as a type of engine especially adapted to the economical working of heavy freight traffic. In 1867, on a number of eight-wheeled four-coupled engines, for the Pennsylvania Railroad, the four-wheeled swing-bolster-truck was first applied, and thereafter nearly all the engines built in the establishment with a two- or four-wheeled truck in front have been so constructed. The two-wheeled or "pony" truck has been built both on the Bissell plan, with double inclined slides, and with the ordinary swing-bolster, and in both cases with the radius-bar pivoting from a point about four feet back from the centre of the truck. The four-wheeled truck has been made with swing-bolster exclusively and without the radius-bar. Of the engines above referred to as the first on which the swing-bolster-truck was applied, four were for express passenger service, with drivers sixty-seven inches in diameter, and cylinders seventeen by twenty-four. One of them, placed on the road September 9, 1867, was in constant service until May 14, 1871, without ever being off its wheels for repairs, making a total mileage of one hundred and fifty-three thousand two hundred and eighty miles. All of these engines have their driving-wheels spread eight and one-half feet between centres, thus increasing the adhesive weight, and with the use of the swing-truck they have been found to work readily on the shortest curves on the road.