The design of a rail depends upon the weight and speed of trains it is to support, their
frequency, and certain influences resulting from the manufacturing process. Data on the various
standard T-rail sections are given in table I. Note that the letters at the top of the eleven
columns to the right on the table correspond to those on the rail section shown in figure 3.1.
The branding symbols given stand for the following rail systems: R.A.-A and R.A.-B, American
Railway Association and American Railway Engineering Association; R.E., American Railway
Engineering Association; and A.S.C.E., American Society of Civil Engineers.
The most generally used designation of rail is its weight, specified in pounds per yard.
Rail which weighs 100 pounds per yard is called 100-pound rail, the "per yard" being understood.
Similarly, rail referred to as 75-pound rail weighs 75 pounds for every yard of length. The
military standard for theater issue is 85-pound rail in 39-foot lengths; 100-and 115-pound rail may
be found on stateside installations.
The weight designation of rail is informative but not completely descriptive. Two rails of
the same weight may vary in section: one may be shorter and have a thicker web or its head may
be narrower and deeper than the other. Their design is influenced by the manufacturing process.
So that rail may be rolled from blooms with satisfactory temperature control, it is
necessary that the amount of metal in the head and flange (base) be approximately equal. The
head must contain enough metal so that the rail can withstand a great deal of wear before it
must be replaced. Modern practice shows that a deep, narrow head is subject to greater end
wear. The base should be wide so that the load is spread over the tie and the rail has a
satisfactory resistance to overturning.
The resistance of a rail to being bent is known as its stiffness. Stiffer rail bends less and
therefore offers less resistance to train movement and reduces maintenance requirements. A
more flexible rail bends more severely under traffic, disturbing ties and ballast.
The stiffness of any beam, whether it is a roof beam or a rail, depends upon its shape
and size. The actual theory of the determination of stiffness is based on calculus, the study of
which is far beyond the scope of this text. Engineers have shown, however, that stiffness is
related to area and height or depth. Since rails are