[8][9] It has also been categorized by the American Public Transportation Association as "Intermodal High Speed rapid rail transit".

The purple color-coded line was formerly known simply as Route 100, but was officially changed to its current name in September 2009 as part of a customer service initiative by SEPTA.

[12] Another crash occurred on July 6, 2012 between Beechwood-Brookline and Penfield stations when the cars detached and came back together, injuring 2.

[15] From Norristown, the P&W RR connected its tracks with the Lehigh Valley Transit Liberty Bell Route to provide direct electric train service from 69th St. Terminal to Allentown, Pennsylvania.

Two years later, the P&W RR was taken over by the Philadelphia Suburban Transportation Company (PSTC), which was more popularly known as the Red Arrow Lines.

In 1956, the Red Arrow abandoned the original branch between Villanova and Strafford, leaving only electric MU train service between 69th Street and Norristown, as it is today.

The PSTC was absorbed into SEPTA in 1970,[3] eliminating the original railroad charter and immediately becoming the "Norristown High-Speed Line Trolley", officially known as Route 100.

By the mid-1980s, the line was in dire condition as the Brill Bullet rolling stock reached over 50 years in service.

[18] In the late 1980s and early 1990s the line underwent a significant modernization program, rebuilding or improving then-dated tracks, electrical substations, the signaling system, bridges, both terminal buildings, and the maintenance depot.

In summer 2013, SEPTA closed the bridge (the Bridgeport Viaduct) carrying the Norristown High Speed Line over the Schuylkill River for four months.

Had the proposed spur to King of Prussia, Pennsylvania been constructed, the proposal would have branded service utilizing the branch as the M3 Montgomery Local to KOP and the M4 Montgomery Express to King of Prussia, providing service between 69th Street Transportation Center, and First & Moore/Valley Forge station, and the M5 Norristown / King of Prussia making local stops between First and Moore/Valley Forge station and Norristown Transportation Center.

[27] Until September 1, 2014, the line used a "pay-as-you-exit" fare collection system on trains towards 69th Street Transportation Center.

As part of a general change on several routes approaching 69th Street, passengers now pay onboard upon entering the train.

Local trains from 69th Street to Norristown stop at all 22 stations, and the trip lasts approximately 30 minutes.

The final environmental impact statement was planned to be submitted to the Federal Transit Administration (FTA) in early 2021.

Construction on the spur to King of Prussia was projected to cost $2 billion and service was expected to begin between 2025 and 2027.