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.
is manufactured in different weights; there are different rail conditions
(wear, corrosion etc) present; there are a significant number of
potential defects possible; and the task has to be performed with
some speed to reliably inspect the thousands of miles of track stretching
across the land. Sperry Rail Service, one of the country's leading
inspector of railroad tracks, has been using specialized test equipment
mounted on self-propelled rail cars for over seventy years to protect
the safety of passengers and freight. This information provides
a brief look at rail inspection.
The history of railroading is rooted in the production of the first
metal rails near the city of Sheffield, England in 1776. The rail improved
the transportation of materials in industries such as mining. In 1803 the
first railroad intended for public use was opened for operation between
the London docks and Croyden. This first railway, the Surrey Iron Railway,
offered a smoother ride than a wagon, but offered no real advantage in
speed since draft animals were used for locomotion. However, the first
steam locomotive was soon to arrive on the scene. In 1804, a steam locomotive
pulled a train of cars carrying several tons of ore for the iron works
at Merthyr Tydfil in South Wales. The first American locomotive, the Best
Friend of Charleston, was placed in operation on the South Carolina Railroad
The rails have evolved from cast iron plates to specially alloyed
steels, which are rolled to a standard shape and specially heat-treated
to obtain the desired properties. The figures above show the progression
of rail development. Present day steel rails are vastly superior
to their predecessors in both strength and wear qualities, however
defects still develop. The heavy loads and high speed of today's
trains can cause rails to fail in service unless regular inspections
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 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
In 1915, the Bureau of Standards began research to determine if magnetic
testing could be used to detect transverse fissures. The inspection technique
involved passing a magnetizing solenoid along the rail to establish a flux
in the rail. Flux leakage caused by a defect was detected with search coils.
The technique was successful in the laboratory but was unable to differentiate
between defects and non-relevant rail features in the field.
1923, Dr. Elmer Sperry, started to develop and build a rail inspection
car with the capability of detecting transverse fissures in railroad
rails. In 1927 Sperry built an inspection car (shown on the right)
under contract with the American Railway Association. The small
flatbed in front of the cab contained the inspection equipment.
The operator and recording devices were housed in the cab.
In 1928, a Sperry built inspection car, SRS 102, was testing rail on
the Wabash Railway in Montpelier, Ohio. The inspection technique Sperry
used established a strong magnetic field in the rail by passing a large
amount of low voltage current through it. A pair of search coils, fixed
at a constant distance from the rail, detected any changes in the magnetic
field around the rail. This magnetic induction flux leakage technique became
the foundation of early rail inspection.
drawing on the left shows the basic operation of rail inspection
using the induction method. Brushes are used to contact the rail
and "inject" electrical current. The current creates a strong magnetic
field in the rail. Where there is a defect in the rail, the steel
material will not support magnetic flux and some of the flux is
forced out of the part. The sensing coil detects a change in the
magnetic field and the defect indication is recorded on the strip
chart. Computers are now being used to record and evaluate the date.
Unfortunately, transverse fissures are not the only types of defects
found in rail. Other manufacturing and service-related defects that can
occur include inclusions, seams, shelling, and corrosion. Fatigue cracks
can initiate from these defects, as well as normal features of the rail
such as bolt-holes. If these defects go undetected, they can lead to rail
head and web separations. Many of these defects are not detectable with
the flux leakage method because the flaws run parallel to the magnet flux
lines or the flaws are too far away from the sensing coils to detect. The
induction technique inspects mainly the railhead.
To complement the flux leakage method, and detect additional flaw types,
ultrasonic inspection has become common. High-frequency sound is transmitted
into the metal rail and reflections from rail joints and surface conditions,
as well as internal defects, are displayed on a screen or cause movement
of a pen on a recording tape. 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 of rail,
like the electromagnetic technique previously discussed. The first all-ultrasonic
inspection car was introduced in 1959. This car was developed specifically
to meet the needs of the New York City Transit Authority (NYCTA).
Fluid filled wheels or sleds are often used to couple the transducers
to the rail. Sperry Rail Services has, over the years, developed
and made use of 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.
Sperry, there are two primary inspection units. The Sperry Rail
Detector Car, referred to as the "big" car, uses both ultrasonic
and electromagnetic technologies to identify defects. The inspection
equipment on a Sperry test car is carried in a carriage slung between
The Hi-Rail trucks currently use only ultrasonics because the electromagnet
equipment is too large for this vehicle. The detector car will test
rail between 6.5 and 13 miles per hour. However, higher speed units
are in development.
data from the inspection equipment is fed to the operator inside
the car. A picture of the operator station is shown on the right.
Federal Railroad Administration (FRA) rules require that any indication
considered suspect by the test equipment on the test car are hand
verified immediately. This leads to a stop-start test mode. When
the operator sees something on the tape indicating a problem, he
uses a buzzer signal system to tell the driver up front to stop.
The car then backs up to the point of examination where the operator
gets out to hand test the rail with an ultrasonic test set mounted
on the rear of the car. If a defect is confirmed, it is marked and
a railroad work crew following the Sperry car will change the rail.
If they can't get to it right away, the section of track is assigned
a slow order (slower speed) until the crew can repair it. The amount
of rail being tested can be increased by the use of chase cars following
the testing vehicles. The chase cars will receive a radioed signal
of the test being done by the lead truck and will stop to do the
necessary hand testing. This elimination of the need to back up
to hand test, allows the testing vehicle to move forward, continuously
testing, with the results being sent and recorded for examination
by the chase car.