The early railroad builders did not neglect the possibilities of the stone bridge. Two notable early examples of this form of construction still remain--the Starrucca Viaduct upon the Erie Railroad, near Susquehanna, Pa., and an even earlier structure, the stone-arch bridge across the Patapsco River at Relay, Md., which B. H. Latrobe, the most distinguished of all American railroad engineers, built for the Baltimore & Ohio Railroad, in 1833-35. The Thomas Viaduct, as it has been known for three-quarters of a century, was the first stone-arch bridge ever built to carry railroad traffic. It was erected in a day when the railroad was just graduating from the use of teams of horses as motive-power. In this day, when locomotives have begun to reach practical limits of size and weight, that viaduct is still in use as an integral part of the main line of the Baltimore & Ohio. It is built on a curve, and consists of 8 spans of stone arches, 67 feet 6 inches, centre to centre of piers, which, together with the abutments at each end, make the total length of the structure 612 feet. It is in as good condition to-day as upon the day it was built. When the Erie Railroad was being constructed across the Southern Tier counties of New York in 1848, its course was halted near the point where the rails first reached the beautiful valley of the Susquehanna. A side-valley, a quarter of a mile in width, stretched itself squarely across the railroad's path. There was no way it could be avoided, and it could be crossed only at a high level. For a time the projectors of the Erie considered making a solid fill, but the tremendous cost of such an embankment was prohibitive. While they were at their wits' ends, James P. Kirkwood, a shrewd Scotchman, who had been working as a civil engineer upon the Boston & Albany, appeared. Kirkwood spanned the valley with the Starucca Viaduct, one of the most beautiful bridges ever built in America. He opened quarries close at hand and by indefatigable energy built his stone bridge in a single summer. It has been in use ever since. The increasing weight of its burdens has never been of consequence to it, and to-day it remains an important link in a busy trunk-line railroad. It is 1,200 feet in length and consists of 18 arches of 50 feet clear span apiece. But stone bridges even then cost money, and so the timber structure still remained the most available. Many men can still remember the tunnels, into whose darkness the railroad cars plunged every time they crossed a stream of any importance whatsoever. They have nearly all gone. The wooden bridge was ill suited to the ravages of weather and of fire--ravages that were quickened by the railroad, rather than hindered. A substitute material was demanded. It was found--in iron.