List of unused or defunct New York City Subway services

The Metropolitan Transportation Authority (MTA) does not use some letters and numbers to identify routes on the New York City Subway.

 Unused letters 

 : the letter I could be confused with Line 1.
 : the letter O could be confused with the number 0, which the MTA uses to help their computers recognize the 42nd Street Shuttle.
 : the letter P could sound funny or rude, because it sounds like the word “pee”.
 : the letter U could be confused with the word “you”.
 : the letter Y could be confused with the word “why”.

 Internal numbers and letters 

The MTA privately (or internally) uses the number 0 and the letter H, to help their computers recognize the three separate shuttles. The three shuttles use the letter S on maps and posters, and this can confuse computers.

 : The number 0 represents the 42nd Street Shuttle.
 : The letter H represents the Rockaway Park Shuttle. The last time that the MTA used the letter H on maps and posters was in 2012, because the damage from Hurricane Sandy prevented the MTA from running Line A.

 Reserved letters 

The MTA reserves the letters T and X to identify the services that they want to build or run in the future.

 : the letter T represents a service that the MTA plans to run, when they finish building the Second Avenue Subway.
 : the letter X represents a service that the MTA plans to run in the future, but have not decided on what number or letter to use.

 Defunct letters and numbers 

 : the number 8 used to represent trains that ran on the IRT Third Avenue Line. Line 8 stopped running on April 28, 1973, when the MTA closed the Third Avenue Line.
 : the number 9 used to represent trains that made fewer stops along the same route as Line 1. Line 9 stopped running on May 27, 2005, because it caused riders to wait too long for other trains on the IRT Broadway - Seventh Avenue Line.
 : the letter K used to represent trains that ran via the IND Eighth Avenue Line, between 168th Street and World Trade Center. Line K stopped running on  December 11, 1988.
 : the letter V used to represent trains that ran via the IND Sixth Avenue Line, between Forest Hills–71st Avenue and Lower East Side–Second Avenue. Line V stopped running on June 25, 2010, because the MTA did not have enough money to operate the service.

 Unused or defunct letters and numbers on trains 

 : Trains of the R62 and R62A type can display the numbers 8, 10, 11, 12, and 13. When the trains first entered service, the MTA was thinking of replacing diamond-shaped symbols with separate numbers to represent fast trains.
 : Trains of the R32 and R32A type can display the letters P, T, U, X, and Y. The extra letters allow the MTA to use any of those letters for a special service, during an emergency.

Trains of the "New Technology Train" family, such as the R142, R143 and R211 types, use digital displays. That means the trains can easily display any service number or letter.