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28 Shared Use of Railroad Infrastructure with Noncompliant Public Transit Rail Vehicles: A Practitioner's Guide 1. Train control systems; 2. Communications technology; and 3. Rules and procedures. Effective integration means that Rules and Procedures are woven into the train control system, in conjunction with a communications network, to assert effective C&C over all train movements while protecting employees and the public. Together they provide redundant fail-safe features to prevent collisions and protect against technical failures or human errors. Each individual facet contributes in a complementary manner with the other two. If one com- ponent of the triangle malfunctions, the remaining two elements must compensate for the defi- ciency. During such eventualities, performance can be permitted to suffer, but safety cannot be compromised, particularly in a shared-track environment. Rules and procedures take on greater importance under such circumstances. Train Control Technology Signal and train control technologies are defined as those technologies directly involved in ensuring the safe movement of trains and preventing collisions. This segment reviews current and emerging train control technologies that can provide fail-safe backup to override inevitable human errors and therefore assist in preventing collisions between trains on the same tracks, and between trains and other encroachments into the clearance envelope of an adjacent train. The applicability and practicality of train control technologies for a shared-track setting are assessed and evaluated. Evolution of train control systems has been propelled by many factors. Accident experience and a desire to avoid financial losses, injuries or fatalities, ultimately have driven technological innovation and influences principles of design. More recently, regulations have assumed a greater role in forcing technology development and deployment. Finally, any system must be proven to serve its intended purpose and satisfy functional requirements. Practically speaking, multiple stakeholders must be satisfied with the design, manufacture, installation, and testing of a train control system. While such issues are generic to any train control systems design, their implemen- tation in a shared-track environment merits special attention. In FRA and railroad parlance, they are considered vital systems. 1) Train Control System Functions Train control systems are designed to prevent three major types of collisions: (1) head-on col- lisions between trains traveling in opposite directions on the same track; (2) flanking collisions for trains moving or standing on a siding when approaching or departing a main line track; and (3) rear-end collisions between trains following one another. However, train separation alone does not mitigate all hazards. Besides train-train collisions, a shared-track system poses some unique risks. Intrusion collisions, where freight equipment intrudes on the active passenger track due to a roll-out, derailment, or shifted-load, are not necessarily detected by the train control system. Secondary collisions between freight and pas- senger equipment (i.e., caused by an intrusion event) may not be prevented by the train con- trol system, so the best course is to prevent the primary (root cause) event. Where track is shared between compliant and light passenger rail cars, the FRA has required fail-safe train separation as a sine qua non for its approval. Figure 1 outlines technological approaches to train control that regulators would find acceptable without excessive scrutiny or burdensome strictures.