One of the major problems that railroads have faced since the
earliest days is the prevention of service failures in track.
As is the case with all modes of high-speed travel, failures of
an essential component can have serious consequences. The North
American railroads have been inspecting their most costly infrastructure
asset, the rail, since the late 1920's. With increased traffic
at higher speed, and with heavier axle loads in the 1990's, rail
inspection is more important today than it has ever been. Although
the focus of the inspection seems like a fairly well-defined piece
of steel, the testing variables present are significant and make
the inspection process challenging.
inspections were initially performed solely by visual means. Of
course, visual inspections will only detect external defects and
sometimes the subtle signs of large internal problems. The need
for a better inspection method became a high priority because
of a derailment at Manchester, NY in 1911, in which 29 people
were killed and 60 were seriously injured. In the U.S. Bureau
of Safety's (now the National Transportation Safety Board) investigation
of the accident, a broken rail was determined to be the cause
of the derailment. The bureau established that the rail failure
was caused by a defect that was entirely internal and probably
could not have been detected by visual means. The defect was called
a transverse fissure (example shown on the left). The railroads
began investigating the prevalence of this defect and found transverse
fissures were widespread.
One of the methods used to inspect rail is ultrasonic inspection.
Both normal- and angle-beam techniques are used, as are both pulse-echo
and pitch-catch techniques. The different transducer arrangements
offer different inspection capabilities. Manual contact testing
is done to evaluate small sections of rail but the ultrasonic
inspection has been automated to allow inspection of large amounts
Fluid filled wheels or sleds are often used to couple the transducers
to the rail. Sperry Rail Services, which is one of the companies
that perform rail inspection, uses Roller Search Units (RSU's)
comprising a combination of different transducer angles to achieve
the best inspection possible. A schematic of an RSU is shown below.