The Association of American Railroads sets the safety, security, and operating standards that allow the interchange of equipment across the continent. They dictate everything from the exact profile of a wheel flange to the formatting of reporting marks. If a piece of equipment or a procedure does not meet their manual of standards, it cannot legally operate in interchange service.
Heard on the job
The car knocker bad-ordered the flatcar because the coupler height was out of AAR spec.
Insider tip
Railroaders treat the AAR rulebook like a bible for mechanical standards; knowing the specific AAR manual section for a defect ends arguments on the rip track instantly.