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Page 152 Ji et al. Intell Robot 2021;1(2):151-75 https://dx.doi.org/10.20517/ir.2021.14

economic burden and loss of public confidence could be caused by the defects on the rail track and the
[1]
failure of rail tracks . It was also reported that rail maintenance workers were injured or lost life during rail
inspection and maintenance operations. Thus, safe railway operations demand effective maintenances
which rely heavily on inspection and monitoring of rail track conditions. The condition monitoring is
fundamentally critical to the safety, reliability and cost-efficiency of the rail operations . There are also
[2]
regulations by governments on the frequency of regular track inspections which traditionally requires a
large amount of personnel and equipment resources. Therefore, the rail track condition monitoring is of
great importance due to the safety, economic and regulatory factors.

Rail defects are normally initiated by loads and stresses applied to rails in longitudinal, transverse, or vertical
directions. The vehicle wheels can apply vertical, lateral, and creep loads to the rails, while bulk stress such
as bending stress, thermal stress, and residual stresses can be applied to the rails as well. Different rail
defects such as rail corrugations, rolling contact fatigue defects, squat defects, shatter cracking, split head,
and wheel burns have their own causes and characteristics, lead to different effects, and thus require
corresponding treatments . Therefore, proper detection and classification of rail defects play an important
[3]
role in effective rail maintenance operations. In practice and research, some common rail defects may be
selected and targeted according to Pareto’s principle as some types of rail track defects are commonly found
such as rail corrugation, transverse cracks, shelling, and wheel burns. The train wheels contact with the rail
track and frictions in between will gradually cause rail corrugation over a period of time where crests,
troughs, and waves remain on the track and will become worse and worse. The rail tracks will have concave
deform on the top due to the corrugations, which also cause the rail track lifespan to shorten and, therefore,
a replacement will be needed. Around the faulty weld joint areas, which could be caused by the difference in
weld material or a manufacturing flaw in the rail, transverse cracks may form. Another cause could be the
welding processes, such as arc welding, pores, inclusions, and misalignments. Around the gauge corners of
the rail tracks, there could be subsurface fatigue, which causes the loss of materials and then shelling defects.
Later on, shelling cracks will develop inside the area which can usually be seen as dark spots on the outside
of the gauge corners. However, in the beginning, the cracks may grow so fast that unforeseen failures appear
before the crack defects are detected. The train wheels may slide quite often on the railway tracks, which will
raise the rail surface temperature to be very high and therefore cause the wheel burn defects, which are
usually found in pairs, opposite of one another on the two rail tracks. The high temperature will drop down
quickly, which makes the rail track in the brittle martensite phase according to material science. Wheel
burns could be found on the surface of rail tracks and might appear similar to squat defects. Wheel burns
are usually found in pairs, opposite of one another on two rails.

Defects and deteriorated conditions on the rail track can normally be seen by human eyes; therefore,
manual inspection by patrollers can identify and locate the defects and monitor the condition. However,
such inspections are labor-intensive and can only be arranged during non-operating hours in order not to
disrupt the regular service operations. Ultrasound testing, magnetic particle testing, radiographic testing,
eddy current testing, and penetrating testing are the common non-destructive testing (NDT) methods to
measure the surface and internal parameters or performance of the tested object without destroying it.
Among these testing methods, ultrasound testing and eddy current testing are more suitable for the train
tracks. The ultrasound testing method is the most effective rail track NDT inspection method. It utilizes the
propagation and attenuation characteristics of sound waves in the medium and the reflection and refraction
characteristics on the interface. It can detect the internal defects and find cracks on bolt holes, head, and
web and the longitudinal crack at the bottom of the rail. Due to rail steel being ferromagnetic, the eddy
currents do not penetrate into the material. Therefore, they flow along the crack side so that the pocket
length of the cracks can be determined in the railhead by eddy current testing. The eddy current testing