The SEPTA Cynwyd Line (pronounced Kin-wood) is a line that SEPTA would rather close and its lowest ridership line. The line covers 2.1 unique route miles after 52nd Street branching off of the Paoli-Thorndale Line, and Amtrak's line to Harrisburg up to Cynwyd.

Train service on the line is extremely limited as of 2024 with just five round-trip trains or cars — All trains operate with just a single Silverliner IV or V car — from Suburban Station to Cynwyd that operate during weekdays only. It is the only SEPTA line to have no service to the Market East or Temple University Station.

The line was suspended from April 9, 2020 to September 7, 2021 due to staffing shortages caused by the COVID-19 pandemic, and from June 12 to June 26, 2023 during the I-95 bridge collapse to allow resources to be used to operate additional trains on higher ridership lines.

The line operates on what remains of the Pennsylvania Railroad's Schuylkill Branch that extended all the way to Pottsville, Pennsylvania via Redding (along an alignment that was designed to compete with the Redding Railroad). This line was electrified as far as Norristown in 1930 but passenger service was cut back to Manayunk West in 1960, before being extended to Ivy Ridge in 1980. On May 27, 1986 poor track conductions and structural concerns for the Manayunk Bridge over the Schuylkill River resulted in service pushed back to its current terminus at Cynwyd. The former Ivy Ridge station was replaced by a new station on the Norristown Line at a significantly lower elevation in 1986 to allow SEPTA to no longer have to run shuttle buses. The tracks north of Cynwyd were torn up with the "temporary" discontinuation to Ivy Ridge made permanent in 2008, allowing the Cynwyd Heritage Trail and Ivy Ridge trail to open on the railroad viaduct. The catenary towers (still carrying electricity on side lines) were kept because they are still part of the power grid.