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Summary 9 Barriers to Implementation Research for this project has highlighted some of the advantages and disadvantages of the shared-track concept. None of the disadvantages is insurmountable, if shared-track is the right fit. Some of the more prominent barriers, however, have subjective elements. These cases require convincing regulators and policy makers that shared-track operations are both pos- sible and advantageous to the public. The tactic requires stronger and more irrefutable objective arguments to overcome the necessarily conservative approach to safety. On one hand, more research on and experience with shared-track operations may be essential to effect a change. On the other, existing impediments are the primary reason that the concept has not been more readily embraced, as evidenced by the number of transit agencies that have opted for conventional rail systems, or other projects that were simply stopped. While there are a number of impediments to broader application of this form of service, risk analysis may be the primary obstacle. The interpretation of risk analysis methodology and results is somewhat esoteric. Validated data to quantify risk is lacking; modeling risk events is a complex affair; some have a natural inclination to dismiss risk concerns while some display a tendency to overstate them. One school of thought places excessive faith in risk management, while another has insufficient faith. The probabilistic aspect does not sat- isfactorily address a "nightmare scenario" event. There is simply less comfort in calculating a one-in-a-billion chance of an accident event every 10 years. Regulators can more easily understand the idea that if an event occurs, passengers are protected. Advancing the Shared-Track Concept Track sharing between mainline trains and light passenger rail cars serves a niche market between commuter rail and a stand-alone light rail system. It is clear that advances in this service concept are contingent upon shared-track operations being affordable, and achiev- able without sacrificing safety. The shared-track transit systems currently operating in San Diego and southern New Jer- sey should be designated as demonstration systems for further development, as templates, for an American approach to shared-track transit operations. Use existing shared-track systems to initiate demonstration or pilot programs for concur- rent operations of light passenger rail car and conventional rail equipment in the United States. A shared-track demonstration project without temporal separation could be useful: To gain experience in design and implementation of such a system; To demonstrate the feasibility and safety of such an operation; and To quantify the benefits of shared-track operations. Results of a demonstration could offer the potential for relief from a significant operating constraint on current temporally-separated operations.