Economic benefits of innovative freight car suspensions: Research program shows that using premium trucks results in better economic performance and improved safety in bulk-commodity service

Railway Track and Structures, July, 2001 by Tom Guins, Ken Rownd

Modest changes to familiar bulk commodity truck designs have been shown to result in significant economic benefits in railroad service.

The Association of American Railroads is sponsoring a program to encourage implementation of better truck technologies into commodity-specific freight services. The process involves assessing needs, publishing a performance specification, soliciting new designs, modeling, and evaluating prototypes by test and economic models. From 1999 until now, this program has been focusing on better economic performance and improved safety for bulk commodity service.

Lowering overall railroading cost

It is in the best interest of the railroads and their customers to lower the overall cost of railroading. To accomplish this, railroads need a strategy to implement new technologies to match the efficiency gains of competitors and meet changing market trends. Purchasing decisions should reflect the true cost of railroad service, including market retention, reliability, and long-term damage to the infrastructure.

AAR's Heavy Axle Load Program has demonstrated that significant benefits can be obtained from modest improvement to conventional freight car trucks. Therefore, the AAR is taking the lead in a new program this year to examine methods to accelerate implementation of these cost-effective new technologies. Under this project, barriers that prevent timely implementation will be studied.

HAL increases stress state

The trend in bulk commodity service is toward heavier axle loads. Although it is tempting to think that an optimum weight on rail exists, the optimum number has changed throughout the years in response to improvements in track strength, maintenance practices, and market demands.

Today, the maximum weight on rail in free interchange service is 263,000 pounds for a conventional car. By agreement between railroads, many trains are standardized on 286,000-pound loading. Loading cars to 315,000 pounds has been proposed. The feasibility of 315,000 pound service has been evaluated by extensive industry test and analysis since 1988. Using existing freight car truck suspensions with heavier axle loads will result in proportionally-larger forces imparted to the track. In cases where suspension capacity cannot prevent undesirable vehicle dynamic motions, the increased loading will be much more than that suggested by mere extrapolation based on weight. The result is a higher stress state for railroad infrastructure.

For example, one railroad is experiencing higher than expected damage to the infrastructure, likely attributed to operation of 286,000-pound bulk cars. High lateral loads have been measured by wayside monitoring even in slight (2.7-degree) curves. The result is broken fasteners, apparently independent of fastener type. Reduced component life has lead to increased maintenance and costs.

Stronger track and more controlled vehicles are needed to mitigate the negative impact of this higher stress state.

Economics or design?

One method to accelerate implementation of better vehicle suspensions is to establish new rules for heavy vehicles that will limit the loads applied to the infrastructure. Older vehicles structurally capable of heavier axle load service also need to be addressed in this approach. Current industry practice focuses on vehicle safety, but does not specify maximum loads into the track. In addition, equipment must be maintained to provide lasting good performance. A method to monitor or sample long-term track friendliness may be needed.

HAL Program - 1988-present

The HAL program run by the Transportation Technology Center, Inc., provides a consistent test bed to compare the effects on infrastructure from changes in track components and freight car equipment. This program generates experience necessary to make maintenance observations concerning axle loads, truck types, and improved track components. This experience cannot be gained in mixed railroad service. The HAL operations at FAST are focused on 315,000-pound conventional freight cars.

A recent economic study compares total direct costs using best-practice track structure available to the railroad industry. This best practice assumes the exclusive use of high-integrity special track work, higher hardness rail in tangent track, and increasing rail hardness in curved track. Studies show that considerable investment in track and bridges needs to be made to realize the overall operating benefits.

One phase of HAL documented the value of improved freight car suspensions. Track costs were reduced to the point that a decision was made to return to conventional trucks to provide a more-severe (and realistic) environment for subsequent track component testing.

HAL provides a direct method to quantify the benefits of a complete change over to better suspension trucks. The program does not provide a practical or timely test bed for a range of improved suspension designs.

Bulk commodity truck program

The bulk commodity truck program has attempted to provide a method to calculate benefits from new suspension designs based upon the performance of a single prototype. This was accomplished by defining test regimes and analysis methods that translate controlled test results to dollars of benefit in industry service.