[2] The 9 operated during rush hour periods from 1989 to 2005, as a variant of the 1, providing service between Van Cortlandt Park–242nd Street in Riverdale, Bronx, and South Ferry in Lower Manhattan.

The 9 was temporarily suspended between 2001 and 2002 due to severe damage to the Broadway–Seventh Avenue Line caused by the September 11 attacks, and was permanently discontinued in 2005 as a result of a decrease in the number of riders benefiting.

The 9 designation was originally used for the Interborough Rapid Transit Company's (IRT) Dyre Avenue Line.

It served the former New York, Westchester and Boston Railway between Dyre Avenue and East 180th Street, connecting to the IRT White Plains Road Line at the latter station.

[4] The shuttle was relabeled SS in 1967,[5] and then renamed as part of the 5, the same as the through service on the line through East 180th Street to Dyre Avenue.

[9] As part of the study that resulted in the skip-stop plan, the NYCTA examined the feasibility of using the center track for express service.

[11] Skip-stop trains would not speed through stations, instead passing through skipped stops at 15 mph (24 km/h), the maximum allowed per NYCTA rules.

Skip-stop service was expected to speed up travel times for almost half of riders north of 96th Street.

[15] In October 1988, the NYCTA informed local communities that it planned to implement skip-stop the following spring; residents of Inwood and Washington Heights were particularly opposed to the change.

[16] In March 1989, the NYCTA stated that there was not a set date for the implementation of the plan, with service possibly starting as late as the fall.

[19] Beginning at 6:30 a.m. on August 21, 1989,[20] the services were coordinated as the 1/9 and both ran between Van Cortlandt Park–242nd Street and South Ferry.

[28] On April 27, 2004, it was announced that New York City Transit was considering eliminating 9 and skip-stop service due to long wait times, and as a result of a decrease in the number of riders benefiting.