No, I'd rather we build subways elsewhere than add tracks to existing lines. It would probably cost the same amount anyway. Outside of New York, most systems in the world don't have express tracks.
Sections of Chicago's system (North Side Main Line), the majority of the Broad Street Line in Philadelphia and the London Underground (mostly in inner suburbs, such as the common track shared by the District and Piccadilly Lines or a section of the Metropolitan) also have four tracks, but yes, New York is the only place where they make lots of use of it.
It's funny that Philly has a four-track subway (at least on the north side), yet it's overbuilt for what they need today. They have express trains still during the peak, yet long waits between them. It's the classic case of having (mostly) all the transit infrastructure they will ever need (between on-paper amazing regional rail network with airport rail link, streetcars, interurbans, commuter subway, four track main line), but not enough demand/service to get full benefit from it. Opposite problem to Toronto.
Here's what it looks like at Girard and Broad, the junction of a streetcar route (the 15) and a four-track local-express subway:
http://goo.gl/maps/U7rl0
A KFC with drive through and parking on one corner, a gas station and full-service McDonald's with drive through on another, a one-story CVS Pharmacy on another, and a two-story 1930s-vintage building on the fourth.
