Improvements in the processes of manufacturing steel, which resulted in the general use of that material for rails and tires, have made it possible to nearly double the weight which was carried on each wheel when they were made of iron. The weight of rails has also been very much increased since they were first made of steel. Twenty or twenty-five years ago iron rails weighing 56 pounds per yard were about the heaviest that were laid in this country. Now steel rails weighing 72 pounds are commonly used, and some weighing 85 pounds have been laid on American roads, and others weighing 100 pounds have been laid on the Continent of Europe. [Illustration: Fig. 27.--Mogul Locomotive. By the Schenectady Locomotive Works.] [Illustration: Fig. 28.--Ten-wheeled Passenger Locomotive. By the Schenectady Locomotive Works.] [Illustration: Fig. 29.--Consolidation Locomotive (unfinished).] [Illustration: Fig. 30.--Consolidation Locomotive. By the Pennsylvania Railroad Company.] Of late years urban and suburban traffic has created a demand for a class of locomotives especially adapted to that kind of service. One of the conditions of that traffic is that trains must stop and start often, and therefore, to "make fast time," it is essential to start quickly. Few persons realize the great amount of force which must be exerted to start any object suddenly. A cannon-ball, for example, will fall through 16 feet in a second with no other resistance than the atmosphere. The impelling force in that case is the weight of the ball. If we want it to fall 32 feet during the first second, the force exerted on it must be equal to double its weight, and for higher speeds the increase of force must be in the same proportion. This law applies to the movement of trains. To start in half the time, double the force must be exerted. For this reason, trains which start and stop often require engines with a great deal of weight on the driving-wheels. In accordance with these conditions a class of engines has been designed which carry all, or nearly all, the weight of the boiler and machinery, and sometimes the water and fuel, on the driving-wheels. For suburban traffic, the speed between stops must often be quite rapid, and consequently the engine must have a long wheel-base for steadiness, as well as considerable weight on the wheels for adhesion. Four-wheeled engines (Fig. 14) have all their weight on the driving-wheels, but the wheel-base is short. [Illustration: Fig. 31.--Decapod Locomotive. By the Baldwin Locomotive Works, Philadelphia.] [Illustration: Fig. 32.--"Forney" Tank Locomotive. By the Rogers Locomotive and Machine Works, Paterson, N. J.]