. Railroad construction, theory and practice; a text-book for the use of students in colleges and technical schools . sary on account of ■evaporation. Such platforms should also be provided as refuge- iBAYS for track-walkers and trackmen working on the trestle. On >very long trestles such a platform is sometimes provided withsufficient capacity for a hand-car. 183. Timber. Any strong durable timber may be used whenthe choice is limited, but oak, pine, or cypress are preferredwhen obtainable. When all of these are readily obtainable,the various parts of the trestle will be constructed of dif
. Railroad construction, theory and practice; a text-book for the use of students in colleges and technical schools . sary on account of ■evaporation. Such platforms should also be provided as refuge- iBAYS for track-walkers and trackmen working on the trestle. On >very long trestles such a platform is sometimes provided withsufficient capacity for a hand-car. 183. Timber. Any strong durable timber may be used whenthe choice is limited, but oak, pine, or cypress are preferredwhen obtainable. When all of these are readily obtainable,the various parts of the trestle will be constructed of differentkinds of wood—the stringers of long-leaf pine, the posts andbraces of pine or red cypress, and the caps, sills, and corbels (ifused) of white oak. The use of oak (or a similar hard wood)for caps, sills, and corbels is desirable because of its greater !strength in resisting crushing across the grain, which is thecritical test for these parts. There is no physiological basis tothe objection, sometimes made, that different species of timber, iin contact with each other, will rot quicker than if only one ,. (To face page 216.) § 185. TRESTLES. 217 kind of timber is used. When a very extensive trestle is io bebuilt at a place where suitable growing timber is at hand butthere is no convenient sawmiU, it will pay to transport a port-able sawmill and engine and cut up the timber as desired. 184. Cost of framed timber trestles. The cost varies widelyon account of the great variation in the cost of timber. Whena railroad is first penetrating a new and undeveloped region, thecost of timber is frequently small, and when it is obtainable fromthe companys right-of-way the only expense is felling andsawing. The work per M, B. M., is small, considering that asingle stick 12X12X25 contains 300 feet, B. M., and thatsometimes two hours work, worth perhaps $1, will finish allthe work required on it. Smaller pieces will of course requiremore work per foot, B. M. Long-leaf pine can be pur
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