The Port Authority Trans-Hudson (PATH) heavy-rail rapid-transit system has a two-decade history of using microprocessor-based equipment as replacements for electromechanical and electrical systems. Until recently, these processors were used for non-vital applications only, such as event recording and train dispatching. With the rebuilding of the World Trade Center terminal, processors were introduced for vital applications. PATH is now in the midst of a processor-based replacement of its entire signal system.

Here we present a review of past, current and future applications of processor-based equipment at PATH. We examine the reasons for conversion and the advantages and disadvantages for design, installation and maintenance. A survey is made of equipment that is removed, added, replaced or retained for processor-based conversion. Interfacing with field appliances, including the continued use of relays for limited applications, is examined. With so much functionality concentrated within a single piece of equipment, new types of failure modes and redundancy become important considerations, as well as the volatility of critical information needed to maintain service. We also expose the problems of testing and cutover of new equipment on a railroad that operates passenger service 24 hours a day, 365 days a year, and training requirements of signal office staff and field forces. Since PATH is under FRA jurisdiction, use of processor-based equipment for vital applications is subject to additional regulations and we look at the programming accommodations for FRA testing.

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