The three great stone bridges that the Pennsylvania has built upon its main line are all four-tracked. Two splendid examples of these span the Raritan River at New Brunswick, and the Delaware at Trenton, New Jersey. The third, spanning the Susquehanna at Rockville, Pa., just north of Harrisburg, is the largest stone bridge in the world. It is over a mile in length, and is composed of 48 arches; 220,000 tons of masonry was employed in its construction.
Concrete viaducts were first employed in interurban electric railroad construction, and latterly they have been brought more to the service of the steam railroad. A splendid example of this very new form of construction exists in the extension of the Florida East Coast Railroad over the keys and shallow waters of Southern Florida, for seventy-five miles between Homestead and Key West. A considerable portion of the line is over the sea.
The Florida keys are like a series of stepping-stones, leading into the ocean from the tip of the peninsula to Key West. They lie in the form of a curve, the channels separating the islands varying from a few hundred feet to several miles in width. Nearly thirty of these islands were used in the construction of the new railroad. More than fifty miles of rock and earthen embankment have been built where the intervening waters are shallow, but where the water is deeper and the openings are exposed to storms by breaks in the outer reef, concrete arch viaducts have been used. These viaducts consist of 50-foot reinforced concrete arch spans and piers, with here and there a 60-foot span.
There are four of these arch viaducts aggregating 5.78 miles in length. The longest is between Long Key and Grassy Key, 2.7 miles, and is called the Long Key Viaduct; across Knight’s Key Channel, 7,300 feet; across Moser’s Channel, 7,800 feet, and across Bahia Honda Channel, 4,950 feet. The material of these islands is coralline limestone. In many places the embankment for the roadway is 8 or 9 feet in height, and the roadbed is ballasted with the same material. The result is one of the finest and safest railway roadbeds in the world.
Across the Delaware River at Slateford, Pa., the Delaware, Lackawanna & Western Railroad is building the largest concrete bridge in the world, a few feet longer than the great structure by which the Illinois Central crosses the Big Muddy River and just 100 feet longer than the Connecticut Avenue Bridge, at Washington, D. C. The Lackawanna’s bridge is 1,450 feet long, with five arches of 150-foot span, and a number of shorter arches. The track is carried at an elevation of 75 feet above highwater; and to find living-rock as a solid foundation for a structure of so great a weight, the abutments and piers were carried about 61 feet below the surface of the ground.
With the bridge-builder at his elbow, the railroad constructing engineer hesitates at no river, no arm of the sea, no deep valley, no wild ravine, no cleft in the mountain-side. He calls to his aid the magic of the men who have made this branch of American practical science famous: a feathery trestle appears, as if by magic. Across its narrow edge the steel rails follow their resistless path.