How Do Railroads Deal With Thermal Expansion?


With the advancement of modern continuous welded rail (CWR), rail networks worldwide have had to cope with thermal expansion, or rail stressing. In short, rails contract at low temperatures and experiences tensile stress, in high temperatures, the rail expands and compresses under the stress. This could lead to heat kinks, which force the track out of gauge and could cause derailments if preventive measures are not taken.

How do railroads deal with thermal expansion? Railroads deal with thermal expansion by heating the rail prior to installation. Therefore, the rail is heated to its rail neutral temperature, being heated, then cooled as the rail is laid. Additionally, various technologies utilizing wayside devices are implemented to detect rail faults. 

Background

When railroads were first constructed in the 19th century, each rail was held together by a fishplate (UK) or joint bar (US), which provided the classic clickety- clack sound when a train passed by. However, maintenance of the joints was tedious, as they had to be properly lubricated and serviced in order to prevent unwanted wear on the rail ends. This was a tedious task administered by track crews, as they had to inspect each joint to ensure that it was maintained to an acceptable standard.

Jointed rail was joined together by a joint bar on either side of the rail, with bolts holding it in place. However, a common issue was that the bolts would oftentimes crack the joint bar, especially under the strain of a heavy train. Furthermore, the joints oftentimes had to be insulated when signal circuits were present, where an epoxy type substance was installed between the rails, strengthening the joints.

The effects of train wheels constantly hitting rail joints.
Jeff Hampton

Jointed rail additionally provided for a bumpy ride for passengers, as each wheel would slam the joint as it passed over. As rail technology progressed and high speed trains began to come to fruition, an alternative to the old practice was duly needed. Although a replacement for jointed rail was on the horizon, it did evoke various benefits.

Although jointed rail was rougher on both the train and track infrastructure, it proved advantageous when the rail reacted to intense temperatures. Installed correctly, there should be a tiny lubricated gap between the rails at the joint bar to handle thermal expansion. However, an instance where the joint does not move would have similar reactions to CWR.

Beginning in the fifties, welded rail was installed on various rail networks and began to prove its worth. It encompassed a much smoother ride for passengers, and was less of a maintenance headache. Furthermore, welded rail reduces friction, and wear on both the rail infrastructure and the train’s running gear. However, due to the lack of joint bars, when the rail expanded due to excessive heat, the rails had no leeway, therefore, they expand and buckle under the pressure. However, various precautions are taken to lessen the chance of the rail being damaged due to thermal expansion.

According to Progressive Railroading, if the force of the expanding rail is not repaired in a timely manner, the rail will have no more room to expand, thus, the rail will begin to buckle, resulting in the rail being out of gauge. If the rail was shifted far out of gauge, it will be troublesome for a train to navigate, thus, causing a derailment. This highlights the importance of preventive measures to detect faults before they become a dire issue.

Installation of Continuous Welded Rail (CWR)

In order to prevent thermal expansion, welded rail is installed at its stress free temperature (SFT), which is usually 90 to 110 degrees Fahrenheit in the U.S., and 27 degrees Celsius (81 Fahrenheit) in the United Kingdom. If the temperature of the rail exceeds these temperatures, the rail could begin to buckle under the high stress. According to Progressive Railroading, the rail must be set at a neutral temperature, however, when the rail is laid, it is impossible to determine the rail’s neutral temperature, therefore, the temperature is set by the railroad determined by the location and its climate.

Prior to the installation of a section of continuous welded rail, the temperature of the rail must be taken to ensure it meets the stress free temperature, if the rail is below the desired temperature, the rail will be heated, causing it to expand gradually. The rail is heated either by a gas burner or a tensioner. A tensioner is widely preferred as it evenly expands the rail, as opposed to the gas burner, where the heat is unevenly distributed. However, the tensioner can only be utilized on certain areas of the rail.

Richard Dyke

To improve the strength of the rail, instead of the standard spikes, concrete sleepers utilize elastic fasteners. These fasteners have the ability to prevent vertical and lateral motions on the rails on the sleepers and prevents the rail from raising from the sleeper in the event of an expansion, thus, preventing derailments. However, railroad spikes and wooden ties are widely utilized with CWR, and are equally as effective.

Repairing Rail Stress

When thermal expansion occurs, the issue must immediately be repaired before a derailment occurs. To repair a rail that has endured thermal expansion, the rail must be cut, heated, and welded together. If a rail is found to exceed its stress level, or has experienced a kink, the rail is reheated to expand the steel. Similar to when the rail is installed, when repaired, the rail must be heated to reach the stress free level. If the rail is beyond repair, the piece of rail is completely replaced.

Preventive Maintenance

According to progressive railroading, there are various mechanisms available to combat thermal expansion and rail stressing.  One device called the Railstress Montior, will report the stress level and temperature of the rail. The device is attached to the rail and utilizes various technologies in its analysis, and provides a detailed report of its finding, which is then sent to a wayside device. The device then sends alerts via an alarm if it indicates an issue with the rail’s integrity. In addition to the Railstress Monitor, various devices with similar technology exist.

Nicolas Taveneaux/Public Domain
Expansion Joints on the French TGV.

In addition to this technologically advanced equipment, the Federal Railroad Administration (FRA) has issued various guidelines, mandating that railroads with CWR have an ample training program in place for the track crews and engineers. These programs pinpoint the issue of rail stress, and familiarize railroad personnel with the anatomy of thermal expansion, how to repair track that has expanded or kinked, and the preventive maintenance implemented to prevent these issues.

Furthermore, expansion joints are placed in between lengthy sections of CWR to allow the track to expand, and are oftentimes present near certain pieces of infrastructure such as bridges, as these areas have a tendency to expand differently than the rest of the continuous welded rail. Unlike jointed rail, these joints are tapered diagonally. These expansion joints are one of the many preventive techniques against rail kinks. With CWR the standard for many country’s rail networks, rail companies will continue to be reliant on these technologies and the companies that develop and provide them. Because of these technologies and the education and training being invested to maintain CWR, this type of rail will continue to be standard practice worldwide.

 

Josef

Lifelong Rail Enthusiast and Owner of Worldwide Rails

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