Railway Track & Structures - April 2008 - (Page 16)

AREMA NEWS Technical Article T he topographical data arefundamental to the block design development and verification processes and need to be as accurate as possible before the design calculations begin. For existing track, this information can be obtained prior to the development of the initial block design using data from a previous or new instrument survey. For new lines or for existing lines where the track geometry is expected to change, the locations and extents of grades, curves, switch points, and station platforms, as provided to the block designer, must be maintained during the construction phase. Failure to keep the topography consistent throughout the life of the project may well result in significant changes to the block design, which cascade to project schedule delays and cost overruns due to rework – impacts that escalate as project work progresses. The geographic data provided to the block design developer must include all of the following: 1. Locations of all vertical curves, including grades between curves 2. Units of measurement, including references and baselines used. Use of distance correction equations to be clearly identified. 3. Location of all horizontal curves, including start and end points of curves, points of spiral. Each curve, including compound curves to be described with degree of curvature, radius, and superelevation. 4. Overturning and comfort speeds for each curve. If data unavailable, formulae to be used for computation of these speeds is required. 5. Limits and location of all civil speed restriction zones such as station platforms 6. Location of all switch points 7. Type and dimensions of all interlockings 8. Overturning and comfort speeds for each turnout. If data unavailable, formulae to be used for computation of these speeds is required. April 2008 A discussion of stand-alone veriﬁcations vs.full development Block design veriﬁcation Part 2 by Peter A. Matiukas, senior systems engineer, Safetran Systems Corporation, member of AREMA Committee 37 - Signal Systems 9. Distances between track centers for the affected track segment 10. Locations of insulated joints 11. Locations of interlocking home signals and associated train stops, if so equipped 12. Locations of bumper posts 13. Locations of derailers 14. Locations of all highway crossings Since block design is based on the performance of a complete system, it is also necessary to address several of the characteristics of the wayside control system, especially the means of train detection and cab signal generation. When the block design function is the sole responsibility of the signal system supplier, these data will need to reflect the actual equipment used in the design. If the block design is performed prior to the final determination of the equipment to be used for train detection and speed generation, any assumptions made during the block design must be clearly identified: 1. Nominal and worst-case time for wayside system detection of a train 2. Nominal time for system to register a track circuit becoming unoccupied 3. Type of signals used for train detection 4. Type of signals used for cab signal transmission 5. Worst-case time for system to modify/remove a transmitted speed command All of the aforementioned data are required to either develop a new block design or to completely verify a previously developed block design. When an independent check is to be performed, simply providing a set of control line diagrams to the party performing the checking is not sufficient. While some of the input criteria can be derived from the end product (control lines), much of the critical data cannot. It is worthy of note that any information gleaned from a set of control lines is based on the assumption that they are correct to some degree. For illustration’s sake, if the original block designer did not have all of the customer’s design criteria at the time of development, any omissions and misinterpretations would be carried into the control line diagrams, causing all data contained therein to become suspect. When an independent verification process is called for from a second party, additional complexities arise that require clear definition of the expectations from the entity performing the verification process. Let us proceed on the assumption that the input data are complete and identical for both the block design developer and the party that is performing the verification. The remaining difficulties now stem from how the data are used. Several different suites of block design programs are presently used in the industry, each of which relies upon an independently derived set of algorithms to perform the calculations. The derivation of each of these algorithms is based on a set of assumptions and philosophies that reflect the experiences of the developer. These assumptions and philosophies are not universal! While the fundamentals of each set of block design programs remain fairly consistent, some of the differences such as in how vertical curves are to be modeled (parabolically or linearly), how rotational inertia is handled, and how jerk-rate limiting is used within the calculations, can result in significant differences in the calculated results. The point here is not to judge whether one suite of programs is “more right” than another, it is simply to bring attention www.rtands.com 16 Railway Track & Structures http://www.arema.org http://www.rtands.com

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Railway Track & Structures - April 2008

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