We cannot enter minutely into the development of the locomotive from the crude machine of Trevithic’s time to monsters of to-day. There has been a progressive growth of locomotives in power and in speed. Our biggest freight engines are so powerful that they cannot be used for pulling alone because they can pull a greater load than the draw-bars of the cars can stand. If placed at the head of a long train they would yank the forward cars loose from the rest. Hence they are placed at the rear of the train to act as pushers or in the middle of the train where half their energy is expended in pushing the cars ahead of them and the other half in pulling the rest of the train. The most powerful steam locomotive of to-day (1921) weighs 342 tons and its tender 107 making a total of 449 tons. Its length is 105 feet and its boiler 8 feet 7⅛ inches in diameter. Its low pressure cylinders (4 feet in diameter) are larger than the locomotive boilers of 50 years ago. Its high pressure cylinders are 30 inches in diameter and the stroke is 32 inches. It may be operated either compound or simple, _i. e._, the smaller cylinders may exhaust into the larger ones or they may take steam direct from the boiler. The tractive effort compound is 147,200 pounds and simple 176,600 pounds and the total horsepower developed is 5,040. Each cylinder drives five coupled drivers, in other words there are twenty power-driven wheels with a pair of trailers and a pair of pilot wheels. Six and a half tons of coal are consumed per hour. In the matter of speed a mile per minute has become common and regular scheduled runs over long distances at an average well above sixty miles per hour have been maintained, but the present tendency is to reduce speed somewhat in favor of safety. There are two inventions that have made possible the high speeds of modern railroad travel: the air brake, which has already been described; and the block signal system. The latter, of which there are a number of different types, being electrical, does not properly belong in this book. Stoking a large freight engine or a high speed passenger locomotive is strenuous work. Three tons of coal per hour is not an uncommon rate of consumption. The fireman on a fast express train gets little rest. To relieve him of this exhaustive work automatic stokers are now being used. These convey the coal from the tender to the fire box and feed the fuel at a uniform rate. In place of solid fuel, oil is extensively used in regions where it may readily be obtained. This simplifies the task of firing the locomotives. There has also been some use of powdered coal which is blown into the furnace in much the same way as oil is. To-day steam locomotives have found a serious competitor in the electric locomotive, which is steadily increasing in favor.