The photo above is the High Bridge in Coatesville, Pennsylvania, and it’s an oddity in at least two ways. It’s the only stone arch viaduct among the high bridges I’m looking at, and that’s part of the oddity, but not all. The bridge is on the old main line of the Pennsylvania Railroad (Coatesville is in Chester County, due west of Philadelphia) and is still in use today, carrying both freight and passenger trains over the Brandywine Creek, a road, and another rail line.
The broad history of rail bridges in the US goes from wood in the early days, to a mixture of wood, cast iron, and wrought iron on stone piers in the mid-1800s, to wrought iron and then steel trusses starting in the 1870s, and then concrete and steel girders in the twentieth century. There are many individual exceptions to that description, but it’s a decent starting point. The High Bridge at Coatesville is a series of masonry vaults, which is an ancient structural form heavily used for railroad viaducts elsewhere. There are very few examples in the US, while this type dominates the rather long Wikipedia page on railroad bridges in the UK. The use of this expensive (for the US) and ancient form might seem right if it were very old, but the Coatesville Bridge was constructed 1902-1904. The progression at this site was an 1832 wood bridge, followed by an 1867 “cast-iron” bridge, followed by an 1890 wrought-iron truss, followed by this stone viaduct. Here’s the cast-iron bridge, which appears to be a double-intersection Pratt truss with wrought iron eyebars for the top and bottom chords, circular-section cast-iron struts for the verticals, and wrought-iron rods for the diagonals:
I would not be at all surprised if all three of the pre-viaduct bridges used the same piers. The tops of the piers here look newer than than the rest of the masonry, as if they were raised for the new superstructure.
So why, after some 70 years of following the general trend in bridges, did the Pennsy go back in time for a masonry viaduct? The short answer is that the chief engineer of the railroad, William Brown, preferred masonry bridges to steel because he felt that their durability and relatively lower need for maintenance justified their greater cost. The long answer has to do with the incredible growth of traffic on railroad in the US between the end of the Civil War in 1865 and the beginning of the twentieth century. Not only were rail lines increasing in number and branches, but single track lines were being double tracked (and in the case of the main lines of the biggest railroad, quadruple tracked or more), and the size of trains was increasing. Passenger carriages increased in size some, but increased much more in weight as they went to all-steel construction; freight cars of all types increased greatly in size and weight. As a result, engines increased greatly in size and weight. These trends slowed down around 1900 – there’s a limit to what you can run on a pair of rails – but a forward-thinking designer would have to wonder how fast a structural design would become obsolete. Masonry viaducts may have seemed like a way around that problem.
There’s quite a bit hat could be said about what the modern picture of the viaduct at the top shows – the effloresce indicating water seepage through the railbed, for example. If you’re interested in analysis of masonry viaducts, I suggest visiting Bill Harvey’s Bridge of the Month series. Rather than me attempting to figure things out based on that one photo, you can learn (as I have) from an engineer who has studied these viaducts at length.
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