The NYC subway system runs on OS/2, IBM’s old PC operating system

The MetroCard system has been in operation since 1993, replacing a token system:

… in a very narrow window in the late 1980s, executives at the MTA were looking to remove tokens from the subway system and replace them with a prepaid card. Waldhauer said, “Thinking about all the operating systems in use [in the MTA], I’d have to say that OS/2 is probably the most robust part of the system, except for the mainframe.” It’s still in use in the NYC subway system in 2019.

Source: blog.adafruit.com