Estimating Deterioration in the Concrete Tie-Ballast Interface Based on Vertical Tie Deflection Profile: A Numerical Study

In ballasted concrete tie track, the tie-ballast interface can deteriorate resulting in concrete tie bottom abrasion, ballast pulverization and/or voids in tie-ballast interfaces. Tie-ballast voids toward tie ends can lead to unfavorable center binding support conditions that can result in premature concrete tie failure and possible train derailment. Direct detection of these conditions is difficult. There is a strong interest in assessing the concrete tie-ballast interface conditions indirectly using measured vertical deflections. This paper seeks to establish a link between the vertical deflection profile of a concrete tie top surface and the tie-ballast interface condition using the finite element analysis (FEA) method. The concrete tie is modeled as a concrete matrix embedded with prestressing steel strands or wires. The configurations of two commonly used concrete ties, one with 8 prestressing strands and the other with 20 prestressing wires, are employed in this study. All models are three-dimensional and symmetric about the tie center. A damaged plasticity model that can predict onset and propagation of tensile cracks is applied to the concrete material. The steel-concrete interface is homogenized and represented with a thin layer of cohesive elements sandwiched between steel and concrete elements. Strand- or wire-specific elasto-plastic bond models developed at the Volpe Center are applied to the cohesive elements to account for the interface bonding mechanisms. FE models are developed for both original and worn concrete ties, with the latter assuming hypothetical patterns of reduced cross sections resulting from abrasive interactions with the ballast. Static analyses of pretension release in these concrete ties are conducted, and vertical deflection gradients along tie lengths are calculated and shown to correspond well with the worn cross sectional patterns for a given reinforcement type. The ballast is further modeled with Extended Drucker-Prager plasticity, and hypothetical voids are applied toward the tie ends along the concrete tie-ballast interface to simulate center binding support conditions. The distance range over which the concrete tie is supported in the center is variable and yields different center binding severity. Static simulations are completed with vertical rail seat loads applied on the concrete tie-ballast assembly. The influences of various factors on the vertical deflection profile, including tie type, vertical load magnitude, center binding severity, cross sectional material loss and prestress loss, are examined based on the FEA results. The work presented in this paper demonstrates the potential of using the vertical deflection profile of concrete tie top surfaces to assess deteriorations in the tie-ballast interface. The simulation results further help to clarify minimum technical requirements on inspection technologies that measure concrete tie vertical deflection profiles.

Rail-to-Earth Resistance Assessment for a Medium Capacity Transit System With Continuous Negative Rails by Potential Measurement

This paper presents the development of a qualitative and quantitative assessment of the resistance to ground for the electrically continuous negative rails of a medium capacity transit line of the Taipei Rapid Transit System. Using synchronous potential measurements at three stations we examine potential profiles to locate potential rail sections with low resistance to ground qualitatively. Also the voltage sag values are used to quantitatively calculate rail-to-ground resistance per unit length. The approach presented in this paper requires only voltage measurements with the traction current as the energization source. Thus, this approach can be performed as a routine maintenance procedure to obtain rail-to-ground resistance values from a system-wide point of view.

Comparison of Energy-Saving Driving Style Strategies

Due to the increasing scarcity of fossil fuels and the climate change, the importance of energy efficiency is increasing. This importance is major especially in areas where the energy consumption is high. Rail transport depicts such an area. The highest proportion of energy consumed in the railway is the so called traction energy. This energy is required for the train run. In the timetable, allowances leave a margin for the driving style of train run. By the selective use of strategies that change the driving style, it is possible to exploit these allowances and reduce the traction energy consumption. The first objective of this study deals with the development of algorithms for energy-saving driving style. First, the necessary input variables of the algorithms based on the literature research and the formulas of train dynamics were determined. Then the algorithms were developed to create different energy-saving driving styles, resulting choose the best result which should be shown as a driving recommendation. The developed algorithms were used in an application example in order to calculate the potential of energy-savings. The example should represent the influence of the input variables for a comparison of different situations. At last the acceptance of the determined driving strategies in practice was investigated. By implementing the design thinking method it was identified that driver advisory systems and training programs are necessary to facilitate energy-saving driving in practice.

Case Studies of PTC-Preventable Accidents

A 1969 collision of two Penn Central train resulted in four fatalities and forty-five injuries. This accident could have been prevented, had some type of train control system been in place. After this accident, the National Transportation Safety Board (NTSB) asked the Federal Railroad Administration (FRA) to study the feasibility of requiring railroads to install some type of automatic train control system that would prevent human-factor caused accidents. Over the next almost four decades, a number of additional accidents occurred, culminating in the January, 2005 Graniteville Norfolk-Southern accident and the September, 2008 Metrolink Chatsworth accident. A little more than one month after the Metrolink accident, Congress passed the Rail Safety Improvement Act, which requires Positive Train Control (PTC). To better explain the positive train control requirements, this paper traces each to a detailed case study. Four different accidents are studied, each being an example of one of the four, core positive train control requirements. Included in the case study is a discussion about how positive train control would have prevented the accident, had it been present. This provides positive train control implementers and other railroad professionals with a better understanding of the factors that have caused or contributed to the cause of the positive train control preventable accidents studied.

Moisture Effects on Degraded Ballast Shear Strength Behavior

Ballast consisting of large sized aggregate particles with uniform size distribution is an essential component of the track substructure, to facilitate load distribution and drainage. As freight tonnage accumulates with traffic, ballast will accumulate an increasing percentage of fines due to either aggregate breakdown or outside contamination such as subgrade soil intrusion and coal dust collection. According to the classical text by Selig and Waters [1], ballast degradation from traffic involves up to 76% of all fouling cases; voids will be occupied by fines from the bottom of ballast layer gradually causing ballast clogging and losing its drainage ability. When moisture is trapped within ballast, especially fouled ballast, ballast layer stability is compromised. In the recent studies at the University of Illinois, the focus has been to evaluate behavior of fouled ballast due to aggregate degradation using large scale triaxial testing. To investigate the effects of moisture on degraded ballast, fouled ballast was generated in the laboratory through controlled Los Angeles (LA) abrasion tests intended to mimic aggregate abrasion and breakdown and generate fouled ballast at compositions similar to those observed in the field due to repeated train loadings. Triaxial shear strength tests were performed on the fouled ballast at different moisture contents. Important findings of this preliminary study on characterizing wet fouled ballast are presented in this paper. Moisture was found to have a significant effect on the fouled ballast strength behavior. Adding a small amount of 3% moisture (by weight of particles smaller than 3/8 in. size or smaller than 9.5 mm) caused test specimens to indicate approximately 50% decrease in shear strength of the dry fouled ballast. Wet fouled ballast samples peaked at significantly lower maximum deviator stress values at relatively smaller axial strains and remained at these low levels as the axial strain was increased.

Modeling Track Geometry Degradation Using Support Vector Machine Technique

Analyzing track geometry defects is of crucial importance for railway safety. Understanding when a defect will need to be repaired can help in both planning a preventive maintenance schedule and reducing the probability of track failures. This paper discusses the data cleaning and analysis processes for modeling track geometry degradation. An analytical data model named the Support Vector Machine (SVM) was developed to model the deterioration of track geometry defects. This paper mainly focuses on the following three defect types — surface, cross level and dip. The model accounts for traffic volume, defect amplitude, track class, speed and other potential factors. Results demonstrate that the proposed analytical data model can have a prediction accuracy above 70%.

Tank Car Top Fittings Protection: Evaluation of Derailment Failure Risks — A Conceptual Study

Top fittings devices on tank cars are subject to damage and failure under derailment conditions, potentially leading to the release of hazardous lading. This paper describes the conceptual development of an objective methodology for evaluating the risk of fittings protection failure and the potential reduction in that risk when mitigating strategies such as improved fittings protective structure are deployed. The methodology captures several key elements that affect fittings survival, including: the speed of derailment initiation, the impact velocity/force spectrum experienced by the fittings protective structure during the event, the strength/structural capacity of the protective structure, and the rigidity of the ground surface. Detailed finite element modeling efforts were employed to capture derailment dynamics and corresponding impact velocity spectra, as well as the strength of multiple protective designs. Future work, including validation, is planned to extend the concept into a detailed methodology.

Fatigue Analysis of Rail-Head-to-Web Fillet at Bolted Rail Joint Under Various Impact Wheel Load Factors and Support Configurations

As one of the weakest locations in the track superstructure, the rail joint encounters different types of defects and failures, including rail bolt-hole cracking, rail head-web cracking or separation, broken or missing bolts, and joint bar cracking. The defects and failures are mainly initiated by the discontinuities of both geometric and mechanical properties due to the rail joint, and the high impact loads induced by the discontinuities. Continuous welded rail (CWR) overcomes most disadvantages of the rail joints. However, a large number of rail joints still exist in North American Railroads for a variety of reasons, and bolted joints are especially prevalent in early-built rail transit systems. Cracks are often found to initiate in the area of the first bolt-hole and rail-head-to-web fillet (upper fillet) at the rail end among bolted rail joints, which might cause further defects, such as rail breaks or loss of rail running surface. Previous research conducted at the University of Illinois at Urbana-Champaign (UIUC) has established an elastic static Finite Element (FE) model to study the stress distribution of the bolted rail joint with particular emphasis on rail end bolt-hole and upper fillet areas. Based on the stress calculated from the FE models, this paper focuses on the fatigue performance of upper fillet under different impact wheel load factors and crosstie support configurations. Preliminary results show that the estimated fatigue life of rail end upper fillet decreases as impact factor increases, and that a supported joint performs better than a suspended joint on upper fillet fatigue life.

Comparison of Transfer Lengths in Pretensioned Concrete Railroad Ties Subjected to Different Magnitudes of Rail Loads

In order to quantify the effect of different reinforcement types on transfer lengths, an extensive study was conducted with the selected group of twelve different reinforcement types. These reinforcements are extensively used to produce concrete railroad ties across the world. These employed twelve (12) different types are of 5.32 mm diameter wires with different surface indent geometries. A research team from Kansas state university visited a PCI certified concrete tie manufacturing plant during January 2013. During the plant visit, four (4) concrete railroad ties were cast for each reinforcement type for a total of 48 ties. Considerable part of the study conducted at the plant was previously published by the authors. However for effective understanding, brief explanation of the tie manufacturing process will be presented in this paper. Strain measuring points were mounted on the bottom surface of a concrete railroad tie during the casting process. Proper measures were taken to safeguard these strain measuring points during loading. Transfer lengths were calculated using these mounted strain measuring points. Transfer length measurements were calculated at the plant, immediately after the application of prestressing forces to the concrete ties. After the casting process, two ties for each reinforcement type were stored at plant location for approximately one year and the remaining two ties (companion ties) for the each reinforcement types were shipped and stored at Kansas state university. Transfer length measurements were again calculated at this stage for all 48 ties. Ties stored at plant location were later subjected to cumulative in-track railroad loading of 85 million gross tons over six (6) months period of time. Whereas, the companion ties stored at Kansas state university were not subjected to any loading. Transfer lengths are calculated and compared at this stage and presented [4] in the past. Ties which were already subjected to 85 million gross tons were further loaded to cumulative total of 236.3 million gross tons and the companion ties stored at Kansas State University were not subjected to any loading. Transfer lengths for the ties (twenty four) that were subjected 263.3 million gross tons were calculated and presented in this paper with detailed explanation. Transfer length behavior under different magnitudes of loading is also presented along with the discussion.

Modeling of Diffraction Coupling in Radio Propagation Inside Tunnel Environments

The overall performance of a Communication-Based Train Control (CBTC) system largely depends on the performance of its Data Communication Subsystem (DCS). The DCS network in almost all CBTC commercial system products marketed in the last decade utilizes radio communications in the open ISM bands (2.4 GHz or 5.8 GHz) to establish the bi-directional data link between the central/wayside and onboard segments. To ensure a stable and sound radio communication, a key question is the number of the wayside Access Points (APs) and locations of their antennas. Radio propagation modeling aims to provide an optimal and reasonably reliable solution to the cited question. The diffraction impact of sharp corners and edges in tunnels on the radio propagation process, however, has not been accounted for in majority of models. The purpose of the present research is to incorporate the effect of diffraction coupling due to sharp edges in tunnel sections which include geometrical discontinuities such as cross-junctions and L-bends through ray-mode conversion. The proposed modeling approach offers sufficient versatility to assimilate a variety of discontinuous geometries involving sharp edges in a tunnel environment. Numerical and empirical results suggest that the model provides an accurate tool for analyzing diffraction effects of tunnel discontinuities with sharp edges on the process of radio propagation.