This post began with the picture above, a striking 1880s railroad trestle over a ravine, just downstream of a dam. The text on the photo itself identifies as “Bridge near Encarnacion” but doesn’t say where that is and a Spanish name in the western hemisphere doesn’t narrow the field down much. The Library of Congress metadata says that the photo was a gift from the State Historical Society of Colorado, but there is no Encarnacion, Colorado. That red herring turns out to actually have some meaning, as described below.
The bridge is on the main line of the Ferrocarril Central Mexicano, AKA the Mexican Central Railway. Encarnacion is on the main line, northwest of Mexico City and due west of Tampico.
The creation of the railroad is a rather blunt story of economic imperialism. The “Mexican Central Railway” was incorporated in the US state of Massachusetts, had Americans as its officers, was largely financed from London, and was built, in large part, to serve the traffic needs of the Atchison, Topeka, and Santa Fe Railway in the US. (The AT&SF was a dominant player in Colorado, and the historical society there probably got the picture from the railroad.) At first, the steel (rails) and iron (bridges) came from the UK, but there were delays, and the breaking point came at the Encarnacion Bridge, where the overall work on the line was delayed because of slow shipments of iron. This incident was important enough to be mentioned in an architectural travelogue a few years later. Unfortunately, the easy-to-access records don’t make it clear if this bridge was the last of the UK imports or the first of the US imports after the company decided that the material had to come from a closer source. I lean towards the latter.
The main span form is a double-intersection Pratt truss: the diagonals are in tension and span two panels as defined by the compression verticals. The form of the members reflects the loading: the tension members are flat bars and the compression members are more complex built-up sections. That is a very American style of bridge – I’m sure such bridges can be found in the UK, but they were built by the hundreds by US railroads.
The deep ravine made it easy to use a deck truss, with the tracks at the top, which simplified the wind bracing and supporting the trusses on the towers below. Deck trusses have some structural advantages and provide better views when crossing, but can look scary. In any case, this a nice example of (a) structure without ornamentation providing a dramatic appearance and (b) unpleasant circumstances behind the construction we see today.
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