The Metro's own PDF talking points document is here. Note that the document does discuss ending late night service but also says that doing so could result in a Title VI discriminatory impact and that savings might be relatively unimpressive, but admits that it could happen Oct. 1
New York City does not run some express trains during off hours and recommends alternate routes sometimes during these periods, but always seems to keep some level of service everywhere. It also has track maintenance advisories, but these seem to have less impact than in DC.
Update: Jan. 22
Was in Philadelphia today. Although the SEPTA system is very complicated, different lines (including streetcar) and different payment systems, at least some of it runs 24x7 on weekends.