Most people who live in Suburban Philadelphia will say that it's the area that follows the R5 train line from the borders of the City in Overbrook, west to Paoli. Some will say it ends in Wayne others will say West Chester even though there's no train in West Chester. Clearly, there's no legal definition as you might have for a zip code or the boundaries of a township or county.
The Main Line is a beautiful place to live no matter where you think it stops. What we do know is that in the early to mid-1800's a train line was planned and built to begin in Philadelphia and end in Pittsburgh.
Homes and estates sprung up along the "Main Line" of the railroad from the 1870's to the 1930's at the time of the depression. The great Philadelphia families started buying large farms and estates from Bryn Mawr to Radnor. Some even went further west.
Today, living along the Main Line is still prestigious. Some of the large estates and parcels of ground still exist but more and more of the McMansions have moved in and large parcels have been developed.