MxV Rail: Lateral Track Strength Increase During Maintenance Speed Restrictions

MxV Rail

PUEBLO, Colo. - From the June 2025 issue of Railway Track & Structures, MxV Rail's Principal Investigator II, Engineering Services, Steve Wilk discusses lateral track strength.

Regular track maintenance is important for preserving track geometry and a healthy track structure against the degradation of passing tonnage. Some of these maintenance activities, such as surfacing, ballast cleaning, or tie replacement, require either moving the tie (e.g., surfacing or tie replacement) or removing ballast (e.g., ballast cleaning). This maintenance, however, will disrupt the interlock between the tie and the ballast particles that helps the ballast hold the tie in place and prevent lateral track movements, such as misalignments and buckles. 

The temporary reduction in lateral track strength (i.e., the strength provided by the ballast against lateral tie movement) immediately after maintenance is expected. Based on this reduction in strength, railroads apply restrictions to the maximum allowable train speed with the goal of reducing vehicle forces until the ballast can recompact and regain its strength capability. Each railroad has a different speed restriction policy, but, in general, all railroads release the speed restriction and resume posted speeds by approximately 0.1 million gross tons (MGT) of accumulated traffic. However, the optimal tonnage accumulation before resuming the posted speed may vary depending on the situation.

Previous studies have investigated the increase in lateral track strength with tonnage but generally focused on tonnage increments at or greater than 0.1 MGT.¹ The goal of this study was to fully characterize the increase in lateral track strength with tonnage after maintenance by filling in the gaps below 0.1 MGT and summarizing the latest study with previous work. The railroads can use this information for data-driven speed restriction policies. This work was supported by the Association of American Railroads (AAR) under its Strategic Research Initiatives (SRI) Program.

LATERAL TRACK STRENGTH

Track ballast has many functions, one of which is to provide restraint against crosstie movement. The lateral track strength, earlier defined as the strength or resistance provided by the ballast against lateral tie movement, is an example of this ballast function. A comprehensive review of the ballast parameters that influence lateral track strength has been the subject of previous publications² and can generally be summarized by the following three components: 1) tie/ballast characteristics, 2) ballast compaction, and 3) amount of ballast surrounding the tie. This article focuses on ballast compaction, often defined as the accumulated tonnage after maintenance, especially where increased ballast density leads to improved tie/ballast interlocking. 

TEST SETUP

Completed in November 2023, the new Facility for Accelerated Service Testing (FAST^®)^[*] track is a 2.8-mile loop that supports railroad research and testing. The section of track used in this test consists of wood ties, cut spikes, and new ballast on a newly constructed fill. The cribs were generally full, and the shoulders were approximately 15 inches wide. A short train consisting of three locomotives and 28 cars loaded to 315,000 pounds gross rail load was used to compact the track. The train speed ranged from 15 to 25 mph.

A common method of directly measuring the lateral tie resistance, Single Tie Push Tests (STPTs) involve unfastening an individual tie and pushing the tie laterally while measuring the lateral force and displacement. During the initial compaction runs of the new FAST track, MxV Rail performed STPTs at increments of 0 MGT, 0.01 MGT, 0.03 MGT, 0.05 MGT, and 0.11 MGT to define the increase in lateral tie resistance as a function of early tonnage accumulation. More test details can be found in other publications.³

TEST RESULTS

The purpose of this test was to fully characterize the increase in lateral track strength with tonnage, and it enabled MxV Rail staff to combine the new test results with three previous studies, all of which focused on tonnage increments above 0.1 MGT.³ Additional tests that focused on 0.1 MGT and 10+ MGT are also included to help define the trend. All data has been normalized by shoulder width and crib height (when data is available) and presented as a percentage of increase from the lateral tie resistance at 0 MGT.

Results

Figure 1 plots 1) the results of the current test (black diamonds), 2) the results of the previous three tonnage increment tests, and 3) the presented equations. The data is split between three panels to emphasize different accumulated tonnage ranges. While there is significant scatter between the tests, likely due to other influences on lateral tie resistance (e.g., ballast characteristics), the results indicate the proposed relationship (thick black line) generally agrees with the current and past test results. Including the 10+ MGT test results (red Xs) with the results from the previous three tests made the trend more conservative, but the additional data from the other tests should make the trend more representative.

The trend of the proposed equation has three parts: below 0.1 MGT, 0.1 to 10 MGT, and above 10 MGT. Below 0.1 MGT, the increase in lateral track strength with tonnage is linear. At 0.1 MGT, the trend becomes non-linear as the rate of increased lateral track strength decreases with increasing tonnage. At 10 MGT, the ballast is generally compacted and the increase in lateral track strength is very low. 

The data also emphasizes the fact that the increase in lateral tie resistance tends to be relative to the 0 MGT lateral tie resistance value. This relationship implies inherently stronger track will see a greater absolute increase from tonnage that can be attributed to differing ballast interlock abilities. Ballast with more interlocking ability will see greater absolute increase in lateral tie resistance from tonnage.

Figure 1. Full trend plotted against available data from multiple sources

Equation Calculation

Due to the different trends, the equation is split into three parts: 0 to 0.1 MGT, 0.1 to 10 MGT, and 10+ MGT. There is significant data at 0.1 MGT and near 10 MGT, so those two increments are used as bounds between the three parts. The non-linear portion between 0.1 MGT and 10 MGT is best fit with a logarithmic trend (similar to track settlement). Table 1 presents the equations for the percent increase in lateral track strength relative to the pre-compaction strength. 

Table 1. Equations for percent increase in lateral tie resistance with tonnage

Table 2 presents the percent of lateral strength increase at different tonnages along with the percent of strength regained from full compaction. Track maintenance does not reduce the lateral track strength to zero. This table emphasizes the fact that speed restrictions are generally lifted when the ballast is partially compacted (~37 percent strength regained from full compaction), as full compaction is not required to safely return to posted speeds.

Table 2. Increase in lateral tie resistance at different tonnage intervals

INTERPRETATION AND CONCLUSIONS

The purpose of the equations in Table 1 is to provide a tool that 1) estimates the lateral strength after ballast maintenance and 2) guides the appropriate speed restriction tonnage. Because compaction is a gradual process, the data show there is no single “tonnage” at which the ballast compacts. The compaction process is generally non-linear, meaning the benefits to ballast compaction from additional tonnage diminish after 0.1 MGT. While 0.1 MGT has been considered a general “rule-of-thumb” and the author agrees with this value as a starting point, the appropriate tonnage may vary depending on issues such as track characteristics, vehicle characteristics, traffic type, geography, season, type of maintenance, and other risk factors. The results from this test and previous tests are based on traffic with heavy-axle loads (e.g., 36 or 39 tons) and track with experiencing ballast recompaction from trains with lighter axle loads (e.g., passenger) may experience lateral track strength increases differently. The influences of these factors on the relationship between accumulated tonnage and lateral track strength have not been defined, but adjustments to the 0.1 MGT rule-of-thumb may be appropriate for specific lines based on the experience of track maintainers. 

Acknowledgments 

The author would like to thank the help from MxV Track Maintenance, Instrumentation, and Engineering teams for their support with the testing and interpretation. The author would also like to thank the “Substructure” and “Track Buckling Prevention” Technical Advisory Groups (TAGs) for test suggestion and direction.

References

1. Kish, A., and G. Samavedam. 2013. “Track Buckling Prevention: Theory, Safety, and Applications.” DOT/FRA/ORD-13/16. Washington D.C.
2. Wilk, S.T. 2023. “Improving Lateral Track Strength After Ballast Maintenance.” International Heavy Haul Association (IHHA) 2023. Rio de Janeiro, Brazil.
3. Wilk, S.T. 2024. “Lateral Track Strength Increase during Maintenance Speed Restrictions.” Technology Digest TD24-003. AAR/MxV Rail. Pueblo, CO.

^[*] FAST® is a registered trademark of MxV Rail.