For almost 40 years Portland Oregon has been developing light rail all over the metro region. Rail has been extended to the major districts within a 10 to 15 mile radius of downtown. (Which is a good share of the "spread" that the regional urban growth boundary allows.)
MAX and street-car lines are heavily used but there are still many cars and traffic can be terrible. So I guess light rail/public transit are never the whole solution by themselves. Then again, what single modality is ever the "magic bullet" answer to enormously complex problems?