Built in 1907 (and extended during the Great Depression), Center City's Market Street tunnel is an example both of the possibilities and the realities of the subway. Originally envisioned as an express subway-local trolley tunnel, the Philadelphia Rapid Transit Corp. (PRT) ran out of money less than halfway through construction from the original 23rd St. portal and was forced to terminate the trolleys at City Hall, resulting in a situation where all subways have to run local east of it, stopping at 13th, 11th, 8th, 5th, and 2nd Streets, while not stopping at all along the axis of postwar skyscraper development along Market west of City Hall.
Never a great fit, even in the best of times, the Market-Frankford Line now stops too frequently, particularly in Market East (where two, rather than three, stops are really called for), or not at all, relative to Center City's major employment and retail destinations*. The trolley transfer at 15th is confusing (you have to access the westbound trolley platform, from 15th, via the Broad Street Line platform), and a full subway stop--not just a local trolley stop--at 19th is necessary to capitalize on the strong demand for Penn Center, Logan Square, and Rittenhouse Square access.
Fixes for this are, needless to say, expensive.
The easiest-to-implement partial "solution" to Market East's too-close-together subway stops is to implement stop-skip service at 11th and 13th, much as occurs elsewhere in the system (particularly on the Frankford Elevated). The free trolley transfer at 13th can also be done at 15th (despite its complexity) and 30th, making it a relatively minor loss if half of the rush-hour trains don't stop there, and the line's stop-skip service has been in use for so long that it's fairly simple just to add an extra "A" and "B" to the map. However--this is still a quick fix, a half-assed solution. And it doesn't remotely address the issue of unmet express-service demand along Market West.
To optimize the tunnel, relative to today's use patterns, would require (a) the consolidation of the 11th and 13th Street stations into a single stop**--certainly feasible from a technical standpoint; the tunnel is mirrored on either side by a pedestrian tunnel; and so opening the pedestrian tunnel to the rail tracks at 12th, along with adding some new fare gates and punching an entrance into the former Reading Terminal's basement, along with some barricades along the former 11th and 13th Street platform edges--nothing too fancy, but nothing ugly--along with a fare-gated passage down to the Juniper station, to maintain the free transfer. This costs some money, of course, but is relatively inexpensive--probably about the same as the already-needed-anyway refurbishment of '70s-garish 11th Street Station.
Market West, on the opposite side, is much more expensive. There the need is to widen the tunnel approximately 10 feet from the western side of the current 19th Street station down to 20th; and were a new 19th Street trolley station elected to be built, two new platform niches for that, too. Like at 30th, the maximal cross-section for a full stop could be as much as 80 feet (Market is 100 feet wide), requiring significant utilities relocation in an underground that is more like Swiss cheese than schist. Furthermore, a new second-level concourse would need to be bored under the rail tunnel; access to an MFL platform without it is impossible. The most cost-effective solution may be to maintain the current 19th Street carstop with a deep new elevator niche and fare barrier*** and access to the sublevel concourse at its western end^, coupled with a brand-new concourse access at 20th^^. Either way, retrofitting an existing four-track subway for a new express stop cannot be cheap.
UPDATE: It occurs to me that this discussion is incomplete without a discussion of 30th Street access improvements. Currently, there is no direct route between the main station and the subway concourse^^^; there used to be one that followed an indirect and winding route, but that has long since been closed, with one entrance being obscured by a pub.
An obvious immediate improvement is to cover the (heavily used) pathway between the main station and subway concourse.
In the longer term, the major access issues are caused by the layout of the area. There is little space to effect direct access; this, indeed, was the reason the original implementation was winding and circuitous, which led to its lack of safety and eventual closure. The only good spot to extend the underground concourse in such a manner as to create a direct connection with the main (intercity) station is currently occupied by the lower level of 30th Street, a route which is also an access road to the coach yards on the north side of the station. The obvious answer to this conundrum is to open a new access path to the coach yards, allowing this access road to be closed and the concourse extended across it towards a direct station connection.
But what if it can't be? Not in the short or medium terms, at least?
A good medium-term plan to offer direct main station-to-concourse access at 30th may be to enclose a pathway connecting the main station to the concourse along the side of this quite wide lower level, with a sort of gated ped-road crossing (as an access road, it's lightly used) where the pedestrian access pathway must needs cross the carriageway. Moveable glass fixtures may be used, allowing the pathway to be climate controlled. No matter what the solution is, however, the space must be well lit and securitized. It must be safe and feel safe. Safety (or the perception thereof) was what did the previous connection in, in the end; just as important as solving access issues is the solving of safety issues in the area.
* Those actually fronting Market Street, remember.
** Why this was not done in the first place, I have no idea. The closest stop to the former Reading Terminal would have been 11th; it is frankly bizarre that there was never direct subway station access. 11th and 13th were obviously built to facilitate department store access (Wanamaker's at 13th; Snellenburg's and Frank & Seder at 11th), but access would still have been possible at 12th, particularly since pedestrian tunnels have been built parallel to the subway from 11th Street west. (These, note, are almost certainly later additions.)
*** Cheaper than a niche for a whole carstop, that's for sure.
^ This setup also has the advantage of making the existing carstop an access concourse for a potential 19th Street subway. The MFL concourse would then terminate in the new line's southbound platform, while the northbound platform would be accessed from the carstop's east end.
^^ This is assuming that the El stop would be built between 19th and 20th Streets. A case can also be made for locating Market West between 18th and 19th Streets, in which case just switch all my "wests" to "easts" and "easts" to "wests" and provision for access to the Penn Center concourse (currently terminating in the BNY Mellon Building at 18th and Market). 19th-20th is more equidistant between 15th and 30th; 18th-19th has better connections access and is slightly closer to the major draws.
^^^ A perception issue as much as anything else. Cities without these sorts of connections just don't quite feel large and worldly--whatever the reality may be. As an example: New York's Grand Central Terminal and Penn Station, Boston's North and South Stations, and D.C.'s Union Station all have these interconnections. Camden and Penn Stations, Baltimore, by contrast, do not.