2.7.
BALLAST SECTION
The ballast section is the cross section of a track between and above the toes (the lowest
points) of the ballast slopes; it may or may not include the subballast. Deciding on the amount,
depth, and disposition of ballast in a ballast section is no less a problem than that of picking the
right material. Again, traffic and terrain govern the decision. In general, heavier, faster trains
require a deeper ballast section than do others. Remember that ballast provides drainage, holds
track in line and surface as permanently as possible, distributes train weight over the roadbed,
reduces dust and weed growth, and provides a means for returning track to line and surface after
passing trains move it. If these purposes are kept in mind, the specifications given below for
depth and the position of ballast will seem reasonable.
To insure that ballast will hold track in place, the crossties are submerged in it until only
the top 2 inches are exposed, as shown in figure 2.3. This anchors the tie in place almost
permanently. Since the track must be adequately drained, the ballast must be deep enough to
drain off all the water falling on or running over the track. Military specifications require a
minimum depth of 4 inches under the tie bottom, but 8 to 10 inches are preferable. In
subparagraphs a and b, details are given on the ballast sections for military main lines and other
less heavily traveled ones.
Figure 2.3. Typical Main-Line Ballast Section
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