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.
Rail 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 in 1831.
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 are performed.
Rail 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 widespread.
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.
In 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.
This 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.
At 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 axles.
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.
The 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.