track--both rails, the accompanying ties, and the tie plates and other hardware--is moved as a unit, this involves a
great deal of work. In addition, after the track is thrown, the ballast must be re-dressed; the shorter the throws the
less ballast work required. Extremely long throws might even make it necessary to widen the ballast section.
It is difficult to set a limit on the size of throws, since so many different things may affect them.
However, the usual half throw should not be more than 24/8, or 3 inches; the full throw would then be 6 inches.
If throws exceed the limit set by policy, the curve should be adjusted. This can be done by the method discussed
in section IV or by adopting a new set of ordinates for the spirals. Usually throws are at a minimum when the
total plus throws are approximately equal to the total minus throws, and neither plus nor minus throws are
bunched in one stretch of track. The conscientious stringliner always tries to achieve this, sometimes revising his
calculations for a curve time and again, until he finally arrives at a smooth curve with small throws.
AN ACTUAL CURVE
One actual railway curve was divided into 36 stations; 22 of them were on the curve itself. The
calculations are shown in figure 3.6. Both tangents were badly out of line while the circular curve varied as much
as 3/4 inch between stations, for example, between stations 7 and 8. Remember, the figures given in the
calculations as well as in the discussion following are in eighths of an inch.
The curve was first computed in the columns headed A in the figure. A smooth curve resulted but the
final station, Sta. 28, required a half throw of +14. The proposed ordinates at stations 4 and 18 were adjusted,
circled in column B under PROPOSED ORDINATE. The B series of computations produced no throw at the
final station but left too large a throw, -31, at Sta. 12, circled in column B under HALF THROW.
The proposed ordinates were adjusted at stations 3, 10, 13, and 19; the new values are circled in column C
under PROPOSED ORDINATE. This change did the job; the half throw at Sta. 12 was reduced from +31 to +24.
The third computation produced a satisfactory curve with reasonable throws so that its results were used.