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Standards for North American railway signaling in the United States are issued by the Association of American Railroads (AAR), which is a trade association of the railroads of Canada, the USA, and Mexico. Their system is loosely based on practices developed in the United Kingdom during the early years of railway development. However, North American practice diverged from that of the United Kingdom due to different operating conditions and economic factors between the two regions. In Canada, the Canadian Rail Operating Rules (CROR) are approved by the Minister of Transport under the authority of the Railway Safety Act. Each railway company or transit authority in Canada issues its own CROR rulebook with special instructions peculiar to each individual property. Among the distinctions are:

In railroad operations, nearly every aspect of employee behavior is governed by operating rules. Employees who perform their jobs in an unsafe manner usually violate operating and safety rules. Human-factors related incidents are caused, or influenced by, unsafe work behavior and attitudes, as opposed to non-behavior related factors like inclement weather or undetected faulty track. Therefore, nearly all human-factor incidents and injuries can be associated with one or more operating or safety rule violations.

By the 1850s, railroad operating rules, often printed as pamphlets or on the back of a time card, had evolved to near universal application. On April 14, 1887 representatives of 48 railroads voted for the adoption of what is now known as the Standard Code of Operating Rules (SCOR), published by the AAR. Thus, all railroad rule books in North America today have as their foundation the SCOR in both development and application.

The SCOR, however, was never intended to be used as a working rulebook. Rather, its primary intention was to standardize operating practices to the extent practicable while still preserving the flexibility of individual railroads to either modify or omit rules at their discretion. Even rulebooks with identical phraseology could be interpreted and applied differently on different railroads. Although used as a reference book, the SCOR was primarily a matrix document, from which the industry could establish standard text and a common numbering system. Until recently, in fact, railroads rarely deviated from the original numbering system.

At present, most Class I railroads in the U.S. use one of two “standard” rulebooks: the Northeast Operating Rules Advisory Committee (NORAC) rulebook and the General Code of Operating Rules (GCOR). Conrail, Amtrak, and several commuter and short line railroads in the northeastern United States use the NORAC rulebook. The GCOR is used by every Class I railroad west of the Mississippi River, most of the Class II railroads, and numerous shortline railroads. A few railroads, including CSX, Norfolk Southern, Illinois Central, Metro North and Florida East Coast, have adopted their own rulebooks. In the case of NS and CSX, the NORAC Rulebook was integrated into their existing rulebook structure with the Conrail merger. Metro-North uses a rulebook based on NORAC. The Long Island Rail Road (LIRR) still uses a rule book that is based on the Standard Code of Operating Rules. Canadian railways use the CROR.

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