Behind the Marker

In the early days, American railroads had to construct bridges that would hold the tremendous and constantly growing weight of the locomotives and rolling stock. At first, wood was the building material of choice, for it was inexpensive, readily available, easy to shape and assemble, and remarkably strong. While stronger and more durable, stone bridges required more money, more time, and tremendous skill to build.

Since the times of the ancient Romans, a masonry arch bridge had consisted of a standard set of components. Wing walls supported the sides, called the "abutments." Spandrel walls faced the roadway on either side of the main arch, also called the "barrel." Voussoir, or "ring," stones formed the graceful arching openings of the bridge, meeting in the center at the "keystone," the critical structural member that bore the enormous pressure of the curving stones. When a masonry arch bridge could be constructed at a perpendicular angle to the roadway or waterway that it crossed, all of these standard features worked well. But for rail lines, such a direct path was often impossible. When a train needed to cross an existing obstacle at an angle or an incline, or along the side of a hill, new forces and design dynamics came into play. Sometimes, too, a roadway needed to pass over a rail line, with the lines of both on differing diagonal paths.



To build bridges that were not perpendicular to the road or track below, masons long ago developed the "skew arch" bridge, in which the two sides of the bridge's archway are offset to form a skewed, out-of-kilter shape. While traditional bridge arches are semi-circular, skewed arches are designed with a more complicated geometry, which required inventive stonework to maintain the needed strength. One way to imagine the design of the stonework in a skew arch would be to cut a diagonal swatch out of a sheet of graph paper, imagining that the squares of the graph are the hewn stones. Curling the paper into a semi circle that represents a skewed arch exposes the difficulty of building an obliquely angled opening. No longer neatly arranged in even rows across the arch, the squares (stones) are distorted and pulled out of alignment. At the ends of the bridge, the builder has to cut the stones on the diagonal, or allow them to protrude in a zigzag pattern. The barrel of the arch is asymmetrical, the two sides of the opening offset. And a keystone is ineffectual, for the load upon it would be uneven because the oblique pattern of the stones will not press equally toward the center.


Forced to cross rugged terrain, early Pennsylvania railroads constructed a variety of masonry skew-arch bridges, one of which still stands in the Allegheny Portage Railroad historical site in Blair County. Built between 1832 and 1834, it is a fine example of the stone masons' skill. Standing at the opening of the arch, one immediately sees clearly what descriptive words such as "offset" and "oblique" really mean.

The magnificent Skew Bridge on Sixth Street in Reading is another fine example of this construction. Erected in 1857, it was designed and constructed under the direction of Richard Osborne, who also designed the first iron railroad bridge in the United States. The large central vault arches gracefully over a wide roadway, and two smaller side arches provide safe walkways for pedestrians. Rumbling across the top, the Philadelphia and Reading Railroad's engines hauled their heavy loads of anthracite coal past Reading's Outer Station, and on to Philadelphia. In 1924, P and R changed its name to Reading Company, which went bankrupt in November, 1971 and was merged into Conrail in 1976. Today, trains of the Norfolk Southern Corporation, its owner since 1999, still use Osborne's rugged skew bridge.