3.25.

ADJUSTING OTHER THROWS

Sometimes a curve includes a point at which the track cannot be moved, such as a bridge, a turnout, or a

restricted clearance. At such a point, the throw or throws must be zero. You must adjust the curve by the method

given in paragraph 3.24 so that they are zero.

At other times, a series of throws may be very large; they can be reduced by the same method. Assume

that you have six half throws that come out to +24, +31, +37, +34, +29, +26. All you have to do is reduce the

third half throw to +25 by choosing a pair of stations 12 numbers apart; the others will become proportionally

smaller. However, when you adjust throws on a curve, always remember to recalculate from the adjustment on to

the end of the curve and to make sure that the last throw is zero.

3.26. SUMMARY

Stringlining is a method of determining the curvature of a rail. By stretching a string tautly between two

points on the inside of the outside rail on a curve, the distance, that is, the ordinate, from the midpoint of the string

to a point five-eighths of an inch from the very top of the rail is equivalent to the degree of curvature. A long

distance indicates a sharp curve; a short one, a light one.

When using a 62-foot string, the length of the ordinate in inches equals the degree of curvature. The

curvature of a correctly lined simple, or circular, curve measures the same throughout. On a spiral, the curvature

progresses evenly.

In the field, you use a tape 31 feet or longer, a string 62 feet or longer marked at the 62-foot mark, a ruler

graduated in inches and eighths of inches, a notebook, a pencil, a crayon, offset blocks, wooden stakes, and

surveyor's tacks.

Follow the steps given here to stringline a curve, including evaluation of the figures and the calculations

involved. Mark and record stations at 31-foot intervals all around the curve. Stretch a 62-foot string tautly

between each set of alternate stations, and measure and record the distance from the string's midpoint to the rail.

Choose a new ordinate for each station so as to insure a smooth curve. The total of new ordinates must equal the

total of measured ordinates. Find and record the difference between the measured and the new ordinates at each

station. If the measured ordinate is greater, the difference is plus; if less, the difference

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