With the advent of steel the possibilities of bridge construction may be said to have become almost unlimited, and their design exceedingly simplified and standardized. EVOLUTION OF THE RAIL. Equally important is the evolution of the rail and its fastenings. The type of metal rails of which the bottom served as the running surface for flat wheels guided by a flange on the rail gave place to "edge" rails on which flanged wheels used the upper surface of the rail before the day of the steam locomotive. Of the edge type, the first were cast iron, fish-bellied, in sections about three feet in length. They were supported by stone blocks or in cast-iron chairs which were in turn made secure to the stone. Later the same type was made of wrought iron by John Birkinshaw, in England, who rolled it up to 15 or 18 feet in length. From 1820 to 1850 the flat strap rail, spiked to longitudinal timbers, in turn supported by cross-ties, was largely used in this country, as it was the only shape that could be rolled here. In 1834 Mr. Strickland designed the Bridge, or "U"-shaped section, which was used on some of our earlier roads and was the first style of edge rail rolled in this country, in 1844. The present "T" section was invented in 1830 by Colonel Stevens, Chief Engineer of the Camden & Amboy Railway, and until 1845, when it was first rolled in this country, had to be imported from England. The poor quality of the iron at this time required such a broad support, in the design of the rail, for the head, that no satisfactory plate fastening could be secured. Iron shoes, into which the rail ends fitted, were the means of connection. The greatest improvement dates from 1855, when the first steel rails were rolled in England. Ten years later they were experimentally rolled here. In 1867, through the introduction of the Bessemer process, which made possible their manufacture at a greatly reduced cost, began a revolution in track construction. While the decade from 1880 to 1890 witnessed the greatest rate of railroad building in this country, it also witnessed the substantial substitution of steel rails on our lines. The earlier rails weighed from 50 to 70 pounds per yard. The increasing weight of equipment brought out a heavier section, and fifteen years ago there was a large percentage of mileage on which weights of 90 pounds and over--and even 100 pounds--per yard had been introduced. Under special conditions rails weighing as high as 140 pounds per yard are used. With the increasing weights of rails, and the development of steel manufacture, greater attention has been paid to details of analysis, process of manufacture, shape and laying, and it may be briefly stated that all these matters are uniformly prescribed at the present time.