The importance of the transition from wood to iron as a structural and bridge building material is generally recognized, but it may be well to mention certain aspects of this change.
The tradition of masonry bridge construction never attained the great strength in this country which it held in Europe, despite a number of notable exceptions. There were several reasons for this. From the very beginning of colonization, capital was scarce, a condition that prevailed until well into the 19th century and which prohibited the use of masonry because of the extremely high costs of labor and transport. An even more important economic consideration was the rapidity with which it was necessary to extend the construction of railways during their pioneer years. Unlike the early English and European railways, which invariably traversed areas of dense population and industrial activity, and were thus assured of a significant financial return almost from the moment that the first rail was down, the Baltimore and Ohio and its contemporaries were launched upon an entirely different commercial prospect. Their principal business consisted not so much in along-the-line transactions as in haulage between principal terminals separated by great and largely desolate expanses. This meant that income was severely limited until the line was virtually complete from end to end, and it meant that commencement of return upon the initial investment was entirely dependent upon the speed of survey, graduation, tunneling, and bridging.
[Illustration: Figure 2.--MODEL OF B. H. LATROBE'S TRUSS, built in 1838, over the Patapsco River at Elysville (now Daniels), Maryland. (_Photo courtesy of Baltimore and Ohio Railroad._)]
The need for speed, the general attenuation of capital, and the simple fact that all the early railroads traversed thickly forested areas rendered wood the most logical material for bridge and other construction, both temporary and permanent.
The use of wood as a bridge material did not, of course, originate with the railroads, or, for that matter, in this country. The heavily wooded European countries--Switzerland in particular--had a strong tradition of bridge construction in timber from the Renaissance on, and naturally a certain amount of this technique found its way to the New World with the colonials and immigrants.