Both of these things can be true at once (shocker, I know). If you coordinate a project with a developer, you can get density in a place that previously didn't have it, but if you don't, you get ugly, low rise sprawl like around Kennedy and Warden. Or Highway 407. Or Pioneer Village, Finch West, or Downsview Park. You bringing up the Flushing line example is something of a red herring, because you make it sound as though the presence of a subway line alone was what resulted in the area being built up, when the reality is far more complex. If you don't have forward thinking people who capitalize on such an asset, you get nothing at all, as at Downsview Park which, nearly a decade after its opening, remains surrounded by a barren, empty wasteland. Your example is a little like pointing to the Prince Edward Viaduct, which was specifically futureproofed by a forward thinking visionary, and assuming that all bridges come futureproofed.