There is perhaps no more striking illustration of the progress of the art than can be obtained from an examination of the illustrations of the various types of locomotives built and operated since 1829. It all bespeaks a tremendous growth, based on a tremendous necessity. We can point to the strengthening of all parts commensurate with the work to be done; to the perfection of detail in materials; manufacture, maintenance and inspection; and possibly observe with pride that the motive power of the railroads of the present contributes an almost negligible part of the difficulties of modern railroad operation, due to features of design or control. CAR CONSTRUCTION. One of the early problems in transportation was to secure the carrying capacity of cars as well as safety. We have pointed out how it was necessary to add a guiding truck to the English locomotive, designed to adapt the same locomotive safely to American conditions. Both the excessive wheel loads on four-wheel freight cars and the greater liability to accident or derailment led at an early time to the use of four-wheel trucks under cars. Between 1831 and 1834 Mr. Ross Winans, of Baltimore, made improvements on cars on the Baltimore & Ohio Railroad. He applied the swivel four-wheel truck, the outside bearing for axles, and the application of the draft gear to the car body and not to the trucks. The increase in lengths of passenger cars, with corresponding increases in weight, led, about 1880, to the quite general employment of a six-wheel truck instead of a four-wheel truck, and even eight-wheel trucks were used for a time, but rejected on account of the excessive length of wheel base and other complications. In 1879 the Allen wheel, consisting of built-up construction with forged-steel tire, was introduced and rapidly became applied to cars in the most exacting service. Originally the tires were imported from the Krupp Works, in Germany, but later were manufactured here. Great interest attaches itself at the present time to the manufacture of solid-steel forged wheels, on account of the reduction in parts. In the latter '80s experiments were made in the development of steel framing for car construction, and built-up steel underframes were introduced shortly after; at first on cars for mineral traffic, where excessive weights and capacities were required. The success of this type of construction has led to its adaptation at the present time to all classes of equipment, and not only steel underframes, but complete steel construction in certain classes of service where the conditions require.