. Railroad construction, theory and practice; a text-book for the use of students in colleges and technical schools . teral deviation. The methods may be somewhat classified aflfollows: ^^ (a) Rimning the line up a convenient lateral valley, turninga sharp curve and working back up the opposite slope. Asshown in Fig. 1, the considerable rise between A and B wassurmounted by starting off in a very different direction fromthe general direction of the road; then, when about one-half ofthe desired rise had been obtained, the line crossed the valley 6 RAILROAD CONSTRUCTION. and continued the climb
. Railroad construction, theory and practice; a text-book for the use of students in colleges and technical schools . teral deviation. The methods may be somewhat classified aflfollows: ^^ (a) Rimning the line up a convenient lateral valley, turninga sharp curve and working back up the opposite slope. Asshown in Fig. 1, the considerable rise between A and B wassurmounted by starting off in a very different direction fromthe general direction of the road; then, when about one-half ofthe desired rise had been obtained, the line crossed the valley 6 RAILROAD CONSTRUCTION. and continued the climb along the opposite slope, (b) Switch-hack. On the steep side-hill BCD (Fig. 1) a very considerablegain in elevation was accomplished by the switchback CD,The gain in elevation from B to D is very great. On the otherhand, the speed must always be slow; there are two completestoppages of the train for each run; all trains must run back-ward from C to D. (c) Bridge spiral. When a valley is sonarrow at some point that a bridge or viaduct of i e/ASonablelength can span the valley at a considerable elevation above tim. Fig. 1. bottom of the valley, a bridge spiral may be desirable. In Fig. 2the line ascends the stream valley past A, crosses the stream atBj works back to the narrow place at (7, and there crosses itself,having gained perhaps 100 feet in elevation, (d) Tunneispiral (Fig. 3). This is the reverse of the previous plan. Ivimplies a thin steep ridge, so thin at some place that a tunnelthrough it will not be excessively long. Switchbacks andspirals are sometimes necessary in mountainous countries, butthey should not be considered as normal types of region must be very difficult if these devices cannot beavoided. .MAP OF AND OF FROM GLEN SUMMIT TO WILKESBARRE ScAle of MUes
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