Two-track subways are the norm generally, at least in the US. In fact, New York's is unusual in that many trunk lines have more than two tracks, mostly separate

Two-track subways are the norm generally, at least in the US. In fact, New York's is unusual in that many trunk lines have more than two tracks, mostly separate express tracks so that express trains can pass locals stopped at a station. (And it is possible to shut down one track of a two-track line for at least part of the way, if you're willing to run only one train through the affected stretch of track at a time. It may condemn you to fifteen or twenty minute waits between trains, but you can do it. The Washington, DC metro has been doing this a lot lately on weekends.)