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.
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.
Reserved letters.
The MTA reserves the letters T and X to identify the services that they want to build or run in the future.
Unused or defunct letters and numbers on trains.
Trains of the R142, R143, R160, R179 and R188 types use digital displays, meaning that they can easily display any service number or letter.