Finally, he was permitted to place it on a Panhandle train, full assurance having been given to the railroad officials that he would be personally responsible for any injury done to their equipment. Four cars and an engine were fitted with the new device and the train started forth from Pittsburgh to Steubenville. On the way its progress was halted by a farm wagon which was caught in the rail at a highway crossing. The engineer whistled for the handbrakes in the good old-fashioned way but he knew that he was too late. Then he thought of the air-brake. He had little faith in the contraption, but he gave its handle a wrench and the train stopped ten feet from the wagon. Several lives were saved and the air-brake was proven. From that day forth it was simply a question of developing the device to its fullest possibility, and Mr. Westinghouse has proved himself able to do that very thing. The air-brake was a fact. Steel had come into use for axles, driving-wheel tires, frames, and every other vital or bearing part of the locomotive; and the designers were again increasing its size. They passed the _Consolidation_ and built the _Mastodon_. These were freighters--each with ten drivers--drivers with tremendous gripping force. They went through what M. N. Forney has called a "period of adolescence in railroad progress," and in that period they experimented with huge driving-wheels only to discard them once again. Then they built bigger engines than even the _Mastodon_; the _Decapod_, with twelve driving-wheels; the _El Gobernador_ which was built by the Southern Pacific at its Sacramento shops in 1884, weighing, with engine and tender fully equipped, 113 tons. Still the locomotive grows and its progenitors talk of the 500-ton machine. They have recently built the Mallet articulated compound, which because of its very great weight has splendid gripping force and is especially adapted for pushing-service on heavy grades. The Baltimore & Ohio, the Erie, the New York Central, the Great Northern, and the Santa Fe have already become committed to this type of engine. The American locomotive Company has just completed for the Delaware & Hudson several Mallet articulated compounds that are among the most powerful locomotives yet constructed. They were designed for pusher service, on heavy grades, north from Carbondale on the main line of the D. & H., which average from .81 to 1.36 per cent. Up to recently the heavy northbound coal traffic up these grades has been handled by the use of two heavy pusher engines. A single one of the new Mallets will do the work of the two pushers, and therein lies the economy in their use.