49 C.F.R. § 238.127
On or after January 28, 2015, low-location emergency exit path marking shall be provided in each passenger car in accordance with the minimum requirements specified in APTA PR-PS-S-004-99, Rev. 2, "Standard for Low-Location Exit Path Marking," Authorized October 7, 2007, or an alternative standard providing at least an equivalent level of safety, if approved by FRA pursuant to § 238.21 . The incorporation by reference of this APTA standard was approved by the Director of the Federal Register in accordance with 5 U.S.C. 552(a) and 1 CFR Part 51 . You may obtain a copy of the incorporated document from the American Public Transportation Association, 1666 K Street NW., Washington, DC 20006, www.aptastandards.com. You may inspect a copy of the document at the Federal Railroad Administration, Docket Clerk, 1200 New Jersey Avenue SE., Washington, DC or at the National Archives and Records Administration (NARA). For information on the availability of this material at NARA, call 202-741-6030, or go to: http://www.archives.gov/federal_register/code_of_federal_regulations/ibr_locations.html
49 C.F.R. §238.127