. Railroad construction, theory and practice; a text-book for the use of students in colleges and technical schools, and a hand-book for the use of engineers in field and office . The value of MN = H has been standardized by the A. R. E. 6i inches for all lengths of switch-rail and for all values of point of the switch-rail (at D) is invariably |-inch it is necessary to calculate MN for other standards ofconstruction, it may be computed (calling aS = length of switch-rail) to be MN=S sin a+(thickness of point of switch rail). 346 RAILROAD CONSTRUCTION. §305. The length to
. Railroad construction, theory and practice; a text-book for the use of students in colleges and technical schools, and a hand-book for the use of engineers in field and office . The value of MN = H has been standardized by the A. R. E. 6i inches for all lengths of switch-rail and for all values of point of the switch-rail (at D) is invariably |-inch it is necessary to calculate MN for other standards ofconstruction, it may be computed (calling aS = length of switch-rail) to be MN=S sin a+(thickness of point of switch rail). 346 RAILROAD CONSTRUCTION. §305. The length to the blunt point of the frog {W = FJ) is given foreach frog in the third column of Table III, Part B. The severalvalues of F and a are also given in Table III. g is the gauge= 4 feet 8i inches = feet. The solution of Eq. 77-80 for various frog angles will give aseries of theoretical leads, as given in Table III. Part closure rails, between the switch points and the frog,will invariably have such odd total lengths that there must beat least one rail cutting (and some wastage of rail) for each ,^1^ *^- F-a A— A-theoretical point of switch rail. j.::::.. Fia. 143. closure length. By shortening the radius of the connectingcurve very slightly and inserting a very short length of tangenteither between the curve and switch-rail at M, or between thecurve and frog-rail at /, all of which will change very slightlythe length of lead, the closure lengths can be made such thatone rail cutting can be eliminated, and yet the combinationsof curves and tangents are mathematically perfect. The detailedmethod of computing these combinations is tedious and willnot be elaborated here, but a series of results developed bythe A. R. E. A. is given under the heading of practical leads in Table III. Part C. § 306. SWITCHES AND CROSSINGS. 347 The above computations and tabular values assume that thetwo switch points (at B and D) are directly opposite. Thiswould always mean that the
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