internal stress in the rail, but this stress for ordinary temperature changes is considerably below
the ultimate stress which rail steel can withstand.
Aside from being a nuisance and an economic liability, broken rails are often the cause of
disastrous derailments and wrecks. The immediate cause of a broken rail is usually an impact
load beyond the strength of the rail. The ultimate source of the failure, however, can usually be
traced to an imperfection in the molecular structure of the steel. This type of imperfection is
particularly dangerous in that it is often inside the rail and therefore invisible. Obviously, some
steps must be taken to prevent such imperfections from leading to rail failure. Many of the
changes in rail form were adopted in an effort to reduce the occurrence of structural
imperfections or flaws.
Rail failures include transverse and compound fissures, split and crushed heads, split
webs, and broken bases. These and other failures, described in the subparagraphs following, are
shown in figure 3.2.
a. Transverse fissure. The most frequent and aggravating of all the flaws is the
transverse fissure, caused by hydrogen remaining in the rail after its manufacture. The fissure--
crack--is a crosswise break, starting from a nucleus inside the head of the rail and spreading
outward. The failure can be recognized by either a bright or dark, round or oval smooth area
around the nucleus. An example is shown in figure 3.2A. Until the rail actually fails, it appears
sound from the outside.
It is common practice to record the heat numbers of all rails put in service. Whenever a
rail fails because of transverse fissure, the heat number is taken and kept on file. After a number
of rails from the same heat have failed, poor steel is suspected. Often, all rails having that heat
number are removed from service.
Electronic detecting devices are used to spot transverse fissures in rails before they fail.
When found, such rails are immediately removed from service. It is unlikely that such devices
would be found in a theater of operations. In those localities, the trackage should be inspected
closely to detect the more advanced cases of fissured rail visible to the naked eye.