The last time I was in DC (December 2007) I quite accidentally stumbled on these remaining streetcar rails on O and P Street in Georgetown. Looks like the city is on a street paving program, but according to this article in Greater Greater Washington, the stones and rails will be spiffed up and put back. No streetcars have run on them since 1960.
There are still working streetcar lines in Philly, Boston and San Fran, among towns I’ve visited. In Brooklyn, streetcars have been left to rust in Red Hook behind a Fairway supermarket.