The Paoli/Thorndale Line, or R5 commonly known as the Main Line, is a SEPTA Regional Rail service running from Center City Philadelphia through Montgomery County and Delaware County to Thorndale in Chester County. It operates along the far eastern leg of Amtrak's Philadelphia to Harrisburg Main Line, which in turn was once the Main Line of the Pennsylvania Railroad and is now part of the Keystone Corridor, a federally-designated high-speed rail corridor.