What you're really saying is those mansion owners could afford horse-drawn carriages or early automobiles. They mostly didn't ride streetcars --which, anyway, mostly didn't go where they lived.I think most of these sorts of homes were built further out into the streetcar suburbs. The precedent of the Boston-Brookline annexation fight made the Boston area an early leader of metropolitan areas where the wealthy spurned inclusion in the city boundaries. You can find a lot of large old 19th century homes in Brookline away from Beacon and Boylston Streets, in Newton, in West Cambridge and, in more "country estate" varieties in places like Milton and Concord.