November 9, 2009
Check this week's classifieds for a range of professional opportunities!
Designing Crashworthiness into Rail Vehicles: It saves lives, prevents injuries, and even benefits the bottom line
BY KAREN HOLMES, Special to Passenger Transport
Advances in the design and engineering of railcars to improve their performance in crashes are attracting heightened attention from public transit agencies and the media. New crashworthiness standards for rail vehicles have been developed and published, and on-the-rails experience is highlighting the difference that crashworthiness features can make in the outcomes of rail collisions. But what exactly is crashworthiness, and how it is designed into rail vehicles?
Crashworthiness Meets the New Science of Crash Energy Management
Simply put, crashworthiness is the ability to protect vehicle occupants during a collision. Crashworthiness features in railcars minimize the risk of serious injury for passengers and operators and can also reduce damage to rail equipment, lowering repair costs.
The U.S. has had safety standards for railcar design since 1939. Initially, these standards mainly addressed the structural strength of railcars. A substantial revision of the early standards was undertaken in 1956 to require improvements in passenger protection, including strengthened car end-posts to reduce the risk of one car climbing on top of the other in a crash (“telescoping”) as well as devices to help keep railcars from moving out of line during a crash (“overriding”).
More recently, the science of designing crashworthiness into railcars has advanced rapidly with the introduction of crash energy management (CEM): a method of designing and manufacturing vehicles in which specific structures within the vehicle are tasked with mitigating the destructive forces unleashed in a collision. Often this involves a controlled, limited collapse within shock-absorbing elements of the railcar, which prevents uncontrolled collapse in the rest of the vehicle.
“One of the most important structural features of CEM design is a crush zone at the front end of a railcar,” said Keith Falk, director of car systems engineering for MTA New York City Transit (NYC Transit) and a member of the Standards Committee on Rail Transit Vehicles organized by the American Society of Mechanical Engineers (ASME) among transit systems, the Federal Transit Administration (FTA), and APTA to develop new crashworthiness standards in transit rail vehicles. “Up to a certain speed, virtually none of the energy of the crash is transmitted to the rest of the railcar,” Falk said.
CEM is made possible by advances in computing technology, which allows engineers to study the likely behavior of vehicle structures and materials in minute detail and thus better predict their behavior during a collision (see sidebar). Under some crash conditions, CEM railcars can protect occupants at collision speeds more than twice that of conventional railcars.
Before CEM, crashworthiness standards were based mainly on the concept of vehicle “buff strength,” which measures the largest force a vehicle structure can sustain without collapsing. However, buff strength alone does not guarantee that vehicle occupants will be protected. Two railcars with the same measured buff strength can perform very differently in a crash.
“With the conventional, buff-strength approach, the collapse could happen anywhere in the railcar,” said Clive Thornes, manager for vehicles engineering with Parsons Brinckerhoff and a member of the rail standards committee. “No distinction is made between the areas occupied by the operator and the passengers, which you want to protect from collapse, and the other parts of the railcar. With the CEM approach, you are trying to limit the collapse to the first two-three feet of the railcar, away from the main passenger compartment.”
Developing Crashworthiness Standards for Rail Vehicles
A new crashworthiness standard for heavy rail transit vehicles that came into effect in mid-2009—ASME RT-2—applies to rail cars used in high-speed, self-contained rail transit systems such as the New York City subway, Washington Area Metropolitan Transit Authority’s Metrorail, and many others. A standard for crashworthiness in light rail vehicles—RT-1—has also been developed and has just been released.
Both standards were developed by ASME’s standards committee, composed of 20 members representing transit agencies, vehicle manufacturers, and other experts. The committee operates on consensus to ensure that the resulting standards will have industry-wide support.
It is still too early to tell what effect the new standards will have on the rail transit industry. “If agencies use the standards and specify them in their procurement of railcars, then the standards will have a positive impact,” said Thornes.
Experience with CEM Design: Heavy Rail
The New York City subway system already has extensive experience integrating CEM design into its railcar fleet. NYC Transit has used CEM-based crashworthiness standards in its procurements since 1999, and all new vehicles must meet those standards.
Of a total fleet of 6,300 vehicles, approximately 3,000 railcars meet CEM performance requirements, according to Falk.
CEM design recently came into play when an empty train crashed in an NYC Transit rail yard, derailing into an adjacent parking lot. “The crashworthiness elements did engage and absorb most of the energy of the crash,” Falk said. “The train happened to be empty but, had there been passengers aboard, the crashworthiness elements would have made a big difference to the outcome.”
Experience with CEM Design: Light Rail
Light rail poses special challenges for crashworthiness design. Light rail vehicles (LRVs) often run on city streets, where there is a greater risk of accidents involving motorists or pedestrians. In fact, collisions between automobiles and LRVs are the most common type of rail accident in the U.S.
To help protect motorists in collisions with LRVs, FTA has supported research to minimize the risk of severe injuries, particularly through modifications to the LRVs’ front end to make them more “crash-friendly.”
“If we can reduce the severity of injury to motorists who stray into the path of light rail vehicles without diminishing the safety of the operator and passengers, then that is all to the good,” said Mike Flanigon, FTA director of safety and security.
The first attempt to design CEM-based crashworthiness into light rail vehicles was for New Jersey Transit Corporation’s Hudson-Bergen line, which opened in 2000. The system’s vehicles feature a specially designed coupler capable of absorbing the energy of a 9 mph collision with no damage to the railcar or its occupants. Behind the coupler are collapsible tubes to absorb the energy of crashes at speeds of more than 9 mph and contain railcar damage in this area.
The latest word in light rail vehicle design can be found in the new METRO system in Phoenix, which entered operation in December 2008. The Phoenix railcars take CEM design a step farther by folding the coupler behind an energy-absorbing bumper that consists of gas shocks, energy-absorbing foam material, and a soft cover.
“The bumper has proven to be highly successful in minimizing damage and possible injury in several collisions involving light rail vehicles and motor vehicles,” said Richard Simonetta, chief executive officer of Valley Metro Rail, operator of METRO.
The innovative bumper design also fares well in bottom-line terms. “In most cases, repair costs have been fairly minor, in the $500 to $1,500 range, though some have been more expensive,” said Larry Engleman, Valley Metro Rail’s director of safety, security, and quality assurance. “Usually, the train is drivable and able to continue in service until it reaches the end of the line. There is no doubt in my mind that repair costs would have been much higher with a conventional bumper,” he added.
Engleman also related an instance in which, he said, the shock-absorbing bumper prevented a death: “A motorist made an illegal left turn into the path of an oncoming train and the car was hit squarely on the driver’s side. If there had been a coupler on the front of our vehicle, as in a conventional railcar, it would have been a battering ram, headed straight into the driver’s compartment. As it was, the elderly gentleman driving the automobile emerged with only a cut on his arm, and was able to walk away from the accident scene, escorted home by a police officer.”
In another instance, the repair cost after a collision was zero. “The repair log simply notes, ‘Scratches buffed out,’” he said.
Experience with CEM Design: Commuter Rail
APTA assisted the Federal Railroad Administration, FTA, and the Volpe National Transportation Systems Center in convening the industry working group that developed the basis for the CEM design for commuter rail. To make CEM work, some exceptions had to be made to FRA regulations for rail vehicles, hence the necessity for broad industry input.
Metrolink in Los Angeles is preparing to receive the first two rail cars designed according to CEM principles—one cab car that can absorb three million pounds of energy in the front and two million pounds in the rear, and one coach car that can absorb two million pounds at either end. According to Bill Lydon, the agency’s director of equipment, Metrolink is the first commuter rail system to do this.
During a period when Metrolink was involved in several serious incidents, Lydon said, the agency became interested in CEM engineering. Lydon himself participated in the development of APTA’s Passenger Rail Equipment Safety Standards.
“After an equipment purchase fell apart, we were in the process of rewriting a specification at the time that one of the incidents occurred,” he said. Metrolink then began examining whether the new one could incorporate CEM technologies. Several federal agencies subsequently helped Metrolink develop that specification for the system’s commuter rail cars.
“This whole effort has proven to be industry-wide among builders, operators, and regulators,” Lydon noted. “From that perspective, it’s an interesting model for going forward with what is really a prototype unit, but there’s a lot of research, study, and actual testing behind it to support what we’re doing.”
The complete Metrolink order is for 117 cars, of which 59 are coaches and 58 are cab cars.
How It’s Done …Using Computer Simulations
Computer simulations play a central role in designing crashworthiness features into railcars, with engineers using the finite element (FE) model to simulate the behavior of complex structures in collisions.
FE models break complex structures into small blocks, referred to as elements. The computer model then assembles all the small elements to predict what will happen to the structure as a whole.
For example, simulating the crush behavior of the cab end of a rail vehicle in a collision with another vehicle may entail 300,000 elements or more, according to Steven Kirkpatrick, principal engineer with Applied Research Associates Inc.
Within the last decade, a complex form of FE model, known as explicit FE, has been applied increasingly often in vehicle design because it enables engineers to better predict the behavior of vehicles in higher-speed crashes.
“At speeds of 5-10 mph, typically there is a small amount of crushing and deformation, a controlled response, which is fairly easy to model. In a 25-35 mph collision, there is more displacement and a much greater risk of structural failure,” said Kirkpatrick.
With the advent of more powerful, affordable computers, explicit FE modeling is now widely used in vehicle design. “The automotive industry really pioneered work in this area, which has now been applied to railcar design,” he said.
A decade of experience with explicit FE models has greatly increased the accuracy of predictions about railcar performance in crashes and reduced the need for expensive physical testing of vehicles by crashing them into walls or into each other. Nonetheless, correlating computer simulation with the results of physical crash tests can still be important.
“If you’ve made incorrect approximations within the computer model, that could produce misleading results,” Kirkpatrick observed. “Comparing the predictions generated by computer simulations with the observed results from crash tests helps to validate the analysis.”
ASME Releases Structural Standards
The American Society of Mechanical Engineers (ASME) Rail Transit Standards Committee recently released structural safety standards for both heavy and light rail transit vehicles. Referred to as RT-2, this heavy rail standard addressing subway-type vehicles was released in 2008 and is now currently in force.
The RT-1 standard for light rail vehicle structures was released in September 2009.
Among its early adopters, Honolulu, HI, may be the first to officially reference RT-1 in the procurement specification for its light rail project.
Copies of the RT-1 and RT-2 standards are available through the ASME web site.
* Applied Research Associates. This web site contains several pages on crashworthiness and transit, including illustrations, movies, and links to research reports.
* Volpe Center paper on “Improved crashworthiness of rail passenger equipment in the United States."
* Martin Schroeder, Chief Engineer, APTA.