In the late 17th century, there was significant Welsh immigration to Pennsylvania for religious and cultural reasons. In about 1681, a group of Welsh Quakers met with William Penn to secure a grant of land in which they could conduct their affairs in their own language. The parties agreed on a tract covering 40,000 acres, to be constituted as a separate county whose people and government could conduct their affairs in Welsh.
Whereas divers considerable persons among ye Welsh Friends have requested me yt all ye Lands Purchased of me by those of North Wales and South Wales, together with ye adjacent counties to ym, as Haverfordshire, Shropshire and Cheshire, about fourty thousand acres, may be layd out contiguously as one Barony, alledging yt ye number come and suddenly to come, are such as will be capable of planting ye same much wth in ye proportion allowed by ye custom of ye country, & so not lye in large useless vacancies. And because I am inclined and determined to agree and favor ym wth any reasonable Conveniency and priviledge: I do hereby charge thee and strictly require thee to lay out ye sd tract of Land in as uniform a manner as conveniently may be, upon yewest side of Skoolkill river, running three miles upon ye same, & two miles backward, & then extend ye parallel wth ye river six miles and to run westwardly so far as this ye sd quantity of land be Compleatly surveyed unto you.—Given at Pennsbury, ye 13th Ist mo. 1684.
TRANSCRIPTION: The "y" is the old letter "thorn". It is read as "TH." The Welsh Tract's boundaries were established in 1687, but notwithstanding the prior agreement, by the 1690s the land had already been partitioned among different counties, despite appeals from the Welsh settlers, and the Tract never gained self-government. The Roberts and other Welsh families became influential in the area, through the building of mills and the eventual introduction of the railroad. It is the railroad that gives the best-known part of the area its current name — the Main Line, named after the mainline of the Pennsylvania Railroad, portions of which were absorbed into Conrail in 1976 as one of the principal rail lines running between Chicago, IL and the Eastern seaboard. After the American Civil War, 104 Welsh families from this region migrated to Knoxville, Tennessee, establishing a strong Welsh presence there. As suburbanization spread westward from Philadelphia in the late 19th century, living in a community with a Welsh name acquired a cachet. Some communities in the area formerly comprising the Welsh Tract were subsequently given Welsh or Welsh-sounding names to improve their perceived desirability. Among these were Gladwyne, formerly "Merion Square", and Bryn Mawr, formerly "Humphreysville".