I'm not sure suburban density is enough. I live in Fairfax County, VA, which has a population density of 3,000 per square mile (about the same as Seattle). They recently extended the Metro out here. Getting to any of the stations, however, is impossible. There are more new Silver Line stations than apartment buildings within a safe and comfortable walk of them (they're surrounded by a scary wide major thoroughfare). I think you'd have to double or triple the surrounding density for them to be worthwhile. I think 8-10k per square mile is the magic number, and there aren't a lot of places in the U.S. that hit that.