moved or "thrown" at each station, expressed in eighths of an inch. If the throw is negative or minus, the track

must be moved in, that is, toward the low or inside rail. If the throw is positive or plus, the track must be moved

out, that is, toward the high or outside rail. When the full throws for all stations have been calculated, the

paperwork of stringlining is complete. Remember, there must never be any throw at the last station.

Sheet 2 of annex A is an overlay showing the same curve after the throws calculated in figure 3.5 have

been made. Place it over sheet 1, annex A, so that stations 0 and 9 coincide on the two sheets. You can then see

that the throwing results in a smooth curve, as intended.

3.23.

MARKED POINTS

Many railway curves are marked with stakes to show where the tangents end, the spirals end, and so forth.

These points are shown in drawings, and may be marked in the field, by letters. As shown in the upper sketch, the

point where the tangent ends and the spiral begins is TS--tangent to

spiral, while the point where the other spiral ends and the tangent

begins is ST--spiral to tangent. The circular part of a curve begins at

SC--spiral to curve, and ends at CS--curve to spiral. As shown in the

lower sketch, if there are no spirals, the curve begins at TC--tangent to

curve--and ends at CT--curve to tangent.

When these points are marked by stakes, they help greatly in

choosing proposed ordinates. For example, the curvature between SC

and CS is constant. The correct proposed ordinate for this stretch is

almost certainly the average measured ordinate, to the nearest whole

number, for the same distance. When ordinates have been assigned to

the circular part of the curve, it is easy to set up smooth spirals between

TS and SC and between CS and ST. Remember, though, that the sum

of the proposed ordinates must always equal the sum of the measured

ordinates. You can adjust your curve