SEPTA operates daily Paoli/Thorndale commuter rail service between Philadelphia and Thorndale on the Philadelphia-to-Harrisburg main line.

The tracks between Harrisburg and Philadelphia are owned and maintained by Amtrak, and are the only part of the Keystone Corridor that is electrified.

The tracks east of Dillerville, just west of Lancaster, were originally the Philadelphia and Columbia Railroad, part of the state-owned Main Line of Public Works.

In 1915, the PRR electrified the line from Philadelphia's Broad Street Station to Paoli, then the west end of commuter service.

Electrification west of Paoli to Harrisburg came in the 1930s, after the PRR completed electrifying its New York-Washington, D.C. section (the present-day Northeast Corridor).

[1] Passenger service remained unprofitable, returned to profitability during World War II, and then slumped again.

The result was dilapidated stations, slow, disjointed track conditions, and antiquated rolling stock which frequently broke down.

Penn Central made an agreement with the federal government to provide a high-speed service called the Metroliner, which upgraded the Northeast Corridor tracks between New York and Washington by 1969, but neglected other areas such as the Keystone Corridor, a lack of maintenance that continued after Amtrak's takeover in 1976.

The Keystone Corridor eventually served as a "depository" for the problem-prone Metroliner electric multiple unit cars.

Due to the slower schedules combined with higher ticket prices and competition from SEPTA, ridership declined.

As on the Northeast Corridor, Amtrak trains between Paoli and Overbrook use the high-speed inner rails for normal operations.

One study suggested two daily electric-train round-trips between New York and Harrisburg with stops in North Philadelphia and Ardmore, a routing last used by Keystone trains in 1994.

A work gang with a track laying system (TLS) installed concrete crossties, new continuous welded rail, and new ballast, allowing for 110 miles per hour (180 km/h).

[10] Between October 3, 2005, and mid-December, Amtrak worked on the number 2 track from Paoli to a point between Narberth and Merion stations.

[11] The busiest part of the Keystone Corridor is the segment between Harrisburg and New York City, which sees multiple trains per day.

Efforts to re-extend the line to Parkesburg and even to Atglen were under discussion by state Congressman Jim Gerlach, R-PA 6, and the Delaware Valley Regional Planning Commission.

[13][14] SEPTA's capital budget for fiscal year 2006 financed an $80.594 million project with Amtrak to improve infrastructure along the line.

[15] SEPTA's effort to improve tracks 1 and 4 between Zoo and Paoli interlockings, included: Norfolk Southern operates overnight freight service between the western junction of the Trenton Cutoff (a former Penn Central electrified "through-freight" line) and just west of Parkesburg via trackage rights, mainly supplying the ArcelorMittal steel plate manufacturing plant in Coatesville.

the Low-Grade Line) and the Philadelphia and Thorndale Branch, were abandoned by Conrail before its purchase by Norfolk Southern, with NS still maintaining the Low-Grade Line due to the catenary poles servicing the Keystone Corridor between Lancaster and Middletown, while the former has been looked at by the Delaware Valley Regional Planning Commission as a possible "Cross-County Metro" project connecting Thorndale with Trenton, New Jersey.

Originally a Pennsylvania Railroad train, this route was discontinued by Amtrak in 1995 but later restarted by the passenger rail company and renamed the Three Rivers.

(Amtrak's current Chicago-to-Washington, D.C. service, the Capitol Limited, uses the rail line west of Pittsburgh).