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The rapid expansion of the American railroads in the past century was made possible by the extensive use of timber in bridge and track construction. The fact that even today about 40 per cent of our total bridge mileage is of timber is evidence that this type of construction is safe, economical, and practical. If all the timber railroad bridges were placed end to end, they would extend for about 1800 miles, which is about the same as the mileage of steel bridges and twice that of concrete bridges. Timber bridges are generally renewed in kind; one large western railroad is renewing timber trestles at the rate of 7000 ft per year.
Ruble, E. J.
Executive research engineer, Association of American Railroads, Chicago, Ill
Drew, F. P.
Research engineer, structures, Association of American Railroads, Chicago, Ill