The 18th Street subway station was originally on the IRT Line, formerly sandwiched between 14th St-Union Square and 23rd Street. After World War II, the Board of Transportation embarked on a platform extension program. The famous City Hall subway station was closed in 1945 because its curved platform could not accommodate the longer trains we see today.
Along this section of the IRT, not only were the platforms extended at 14th Street and 23rd Street, but new entrances were also built at 15th Street and 22nd Street. As a result of the close proximity, 18th Street was closed down in 1948. According to Joseph Brennan of Abandoned Stations, the underground stop was often used for publicity photos because it possessed no unusual features–except for a glass ceiling and decorative motifs designed by Heins & LaFarge, who also worked on the City Hall station. The sealed off station is easily visible when riding the 6 train (and 4, 5 trains if no local trains obstruct the view) between 14th Street and 23rd Street.
Read about more abandoned subway stations in NYC. Additional reporting by @untappedmich