. Railroad construction. Theory and practice . main-tenance. (d) Placing a wedge under the outer rail at each tie. Thisrequires but one extra piece per tie. There is no need of awedge under the inn^r tie in order to make the rail normal tothe tread. The resulting inward inclination is substantially thatproduced by some forms of rail-chairs or tie-plates. The spikes(a little longer than usual) are driven through the wedge intothe tie. Sometimes ^lag-screws are used instead of experience proves that the superelevation is too much or toolittle, it may be changed by this method with less
. Railroad construction. Theory and practice . main-tenance. (d) Placing a wedge under the outer rail at each tie. Thisrequires but one extra piece per tie. There is no need of awedge under the inn^r tie in order to make the rail normal tothe tread. The resulting inward inclination is substantially thatproduced by some forms of rail-chairs or tie-plates. The spikes(a little longer than usual) are driven through the wedge intothe tie. Sometimes ^lag-screws are used instead of experience proves that the superelevation is too much or toolittle, it may be changed by this method with less work thanby any other. (e) Corbels of different heights. When corbels are used (see § 144) the required in-clination of the floor sys_tem may be obtained byvarying the depth of thecorbels. (f) Tipping the wholetrestle. This is done byplacing the trestle on aninclined foundation. Ifvery much inclined, thetrestle bent must be se-cured against the possi-bility of slipping sidewise,for the slope would be considerable with a sharp curve, and the. Fig. 84. 168 RAILROAD CONSTRUCTION. § 148. vibration of a moving train would reduce the coefficient offriction to a comparatively small quantity. (g) Framing the outer posts longer. This case is identicalwith case (a) except that the axis of the system of posts isinclined, as in case (/), but the sill is horizontal. The above-described plans will suggest a great variety ofmethods which are possible and which differ from the aboveonly in minor details. 148. Protection from fire. Trestles are peculiarly subject tofire, from passing locomotives, which may not only destroy thetrestle, but perhaps cause a terrible disaster. This danger issometimes reduced by placing a strip of galvanized iron alongthe top of each set of stringers and. also along the tops of thecaps. Still greater protection was given on a long trestle on theLouisville and Nashville R. R. by making a solid flooring oftimber, covered with a layer of ballast on which the ties an
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