Though it now tunnels in a straight line directly beneath Philadelphia City Hall, prior to 1936, the original MFL trackage between 15th and 13th Street stations separated and looped around the foundation of City Hall (eastbound trains passed around the south side and westbound trains passed around the north side).

[8] The total cost, including road and equipment expenditures, of the Market Street subway and elevated was $23,072,114 (equivalent to about $782,401,000 in 2023).

[11] But the Philadelphia City Solicitor determined that the connection could not be built until a contract for operation had been signed and approved by the PSC.

Total expenditures by the city for the Frankford El "with its track, substations, equipment and certain rolling stock" was $15,604,000 to December 31, 1929.

As I-95 was built through Center City Philadelphia in the late 1970s, part of the Frankford El was relocated to I-95's median, and the Fairmount station was replaced by Spring Garden, on May 16, 1977.

The new Frankford Elevated was built with new stringers and deck installed on the original columns, thus giving not only a reduction in cost, but also reducing the street-level impact on adjoining neighborhoods.

The basic design of the bearings of the reconstructed Frankford Elevated, however, was not appropriate for the repetitive loading from the train traffic.

The problem was first discovered in 1997, but at that time was simply attributed to faulty construction, without evaluation of the root cause.

[29] After Bridge-Pratt closed, the station platforms and the remaining unrebuilt elevated structure above Frankford Avenue and Bridge Street were demolished.

In November 2011, the Federal Transit Administration (FTA), through its competitive Fiscal Year 2011 Sustainability Initiative, awarded $1.4 million to SEPTA to install a "wayside energy storage system" on the Market–Frankford Line.

The plan stated that an infill station on the Market-Frankford Line in this part of the city would not only provide better access for major developments, but it would also create transfer opportunities with frequent north-south bus routes on 19th and 20th Streets.

However, the 2021 Transit Plan said that while initial studies showed such a station would be feasible and highly beneficial, it would be very difficult and expensive to build.

Notable features include being built with Pennsylvania trolley gauge of 5 ft 2+1⁄4 in (1,581 mm),[37][38] and in its use of bottom-contact or underrunning third rail.

[39] Its advantages include a reduced risk of electrocution for track workers and fewer disruptions due to icing conditions during winter weather.

While those systems' elevated lines were built with rails laid on ties (sleepers) that were bolted directly to large steel girders, the Market-Frankford's structure consisted of steel girders supporting a concrete trough deck, which then supported the more conventional railroad construction of rails laid on floating ties with loose rock ballast.

The base fare for riding the line is $2.00 using the Travel Wallet on a SEPTA Key card and $2.50 using a Quick Trip.

The Market–Frankford Line required 142 vehicles at peak hours, cost $86,644,614 in fully allocated expenses, and collected $54,309,344 in passenger revenues, for an impressive farebox recovery ratio of 63 percent.

[47] On February 11, 2008, SEPTA expanded morning and afternoon weekday service with off-peak trains running every six minutes instead of eight.

Weekend late-night hours—5 a.m. on Friday to 1 a.m. on Monday morning—were reinstated on June 20, 2014, a change made permanent on October 8, 2014.

[48] The Frankford Elevated portion opened in 1922 along with another set of cars, numbered 501–600, also built by Brill that year, later receiving the designation Class A-15.

[48] It is currently unknown when these cars were withdrawn, but all had been removed from SEPTA property by the 1970s, with none reported to have been saved for museums.

During the late 1980s and early 1990s, some M-3s were re-gauged to work on the Norristown High Speed Line during the delivery of the N-5 cars.

The M-3 cars were approaching the end of their expected useful lifespan, as well as being increasingly scrutinized for their shaky ride quality and lack of air conditioning.

[54] These cars, designated Class M-4, were manufactured by Adtranz (later Bombardier, now Alstom) at the Dandenong rolling stock factory in Australia and shipped to Elmira, New York for their final assembly.

[55] Delivered between 1997 and 1999, these cars are equipped with AC traction motors, air conditioning, LED signage, and automated announcements.

In February 2017, SEPTA temporarily removed about 90 M-4 cars from service after inspections revealed cracks and signs of fatigue in load-bearing bolsters and associated components.

[57] The Federal Transit Administration gave SEPTA $317 million in February 2024 to fund the purchase of the new rolling stock.

[61] On December 26, 1961, one man died and 38 others were injured when four cars of a train derailed while rounding the curve just north of York-Dauphin Station.

[62] On March 7, 1990, four people died and another 162 injured when the rear three cars of six-car train #61 derailed after leaving what was then 30th Street station westbound at 8:20 a.m.

[66] On September 16, 2017, the front car derailed at Spring Garden Station heading to Frankford TC, nobody was severely injured, but one person was treated for heat exhaustion during the evacuation.