High Quality Content by WIKIPEDIA articles! The Main Line is an unofficial region of suburban Philadelphia comprising a collection of affluent towns built along the old Main Line of the Pennsylvania Railroad (currently Amtrak's Keystone Corridor and SEPTA's R5 line) which runs northwest from Center City Philadelphia. The rail line, from which the area affectionately got its name, was central to creating the Main Line communities which in the 19th century became home to many sprawling country estates built by Philadelphia's wealthiest families.