The trouble is, trains require a certain density to be economic and to run a frequent enough service to be convenient. The ideal city density for an efficient metro service is roughly equivalent to streets of 4 or 5 storey apartment blocks. If you've ever been to Barcelona then you'll have an idea of the ideal density. Very few urbanised areas of the US meet this requirement and most of the areas that do fit the bill date back to before the 1920's and already have mass transit. Buses can operate in cities of lower density, but still require a certain critical mass. I don't have the figures to hand but there has to be a certain number of households within 5 minutes walk of a bus stop to make it viable and many suburbs aren't dense enough for this. See the book 'Cities for a small planet'. Also the Reith lectures on this topic: http://www.bbc.co.uk/programmes/p00gxns8