180 BULLETIN 711, U. S. DEPARTMENT OF AGRICULTURE. 
Topography largely fixes the locations, but the general plan of 
logging determines whether the railroad lines, especially the spurs, 
shall follow valleys, ridge faces, or the tops of ridges. Spurs are 
generally constructed wherever necessary to bring the timber within 
economic yarding, swinging, or roading distance of the track. 
In most cases the main line enters the tract at the lowest point, 
following water grade wherever possible, since this location usually 
gives the best grade out of the region and permits the logs to be 
hauled down grade. Obviously, this is not possible in all cases, 
especially where the topography of the country is such that the 
spur railroads are laid on ridges and the timber yarded up grade. 
The shortest possible route, naturally, is selected, unless heavy cuts, 
fills, bridge work, and trestle work can be avoided by a longer line. 
Switchbacks are frequently used to overcome sharp changes in the 
grade. The same end may be gained more satisfactorily by doubling 
back With a curve. This latter method, however, often necessitates 
a heavier construction expense. Switchbacks often are the only 
practical means at hand for securing timber from elevations above 
or below the main line. When the rise in elevation is considerable, 
it may be advisable to put in an incline. 
Under ■" Ground yarding," it was pointed out that where the 
ground is practically level the spur railroads are located so as to 
gridiron the tract, the distance between the spurs depending for 
the most part on the cost of the roadbed and the stand of timber 
per acre. Figures 16 and IT show how the spur roads may be located 
in practice. In the first case the conditions from the standpoint of 
railroad construction approach the ideal. In the second case the 
ground conditions were harder than usual, which resulted in the 
timber being yarded and swung relatively long distances to the 
railroad. Spur railroads are located by the foreman or superin- 
tendent or by a logging engineer. 
Grades and curvature. — The maximum grades and curvature 
allowed on any logging railroad vary with the character of the road 
and the type of locomotives used. Heavier loads must be hauled on 
the main lines than on the spurs, hence the grades and curvature 
must be lighter. If a geared locomotive is used, they may be heavier 
in either case since a geared locomotive can negotiate heavier grades 
and sharper curves than a rod engine. 
The direction of the traffic and whether or not there are adverse 
grades must be considered in a discussion of grades and curvature. 
Traffic on logging railroads moves only one way, the general direc- 
tion, as a rule, being down grade. Not infrequently, however, ad- 
verse grades are found on these roads, and they limit the number 
of cars that can be hauled at a trip. The effect of grade on the num- 
