"History of the NHRR". New Haven Railroad Historical and Technical Association. 1998. Archived from the original on August 8, 2011. Retrieved July 26, 2013.
"SIGNAL ASPECT of METRO NORTH". RAILROAD SIGNALS of the U.S. September 20, 2020. Retrieved April 9, 2023. Absolute Block Signal is when the Cab Signals fail on the train, showing block is clear to the next interlocking.
"History of the NHRR". New Haven Railroad Historical and Technical Association. 1998. Archived from the original on August 8, 2011. Retrieved July 26, 2013.