The idea's been talked about for years, but it’s one of those things like "widen / double-deck the Expressway" - intriguing but highly complex once you get into the weeds..
First, the use of the same "gear" was only a stopgap measure during the infamous period between demise of the Brill cars and arrival of the N-5s. SEPTA took half a dozen (IIRC) single-unit Almond Joys and extensively modified them for use on tripper runs limited to the lower end (69th St. - Bryn Mawr) of the line. The cars needed modification to accommodate station clearances, signaling, and fare collection.
But the real impediment is that it wouldn’t be a simple matter to reconfigure an interurban line for subway trains. The El and NHSL (P&W) have different geometries, platforms, and operating characteristics.
• The NHSL couldn’t simply be regauged, it would have to be reconfigured from the roadbed up to deal with platform clearances and curve radii. Platforms would have to be lengthened and some possibly even moved because they’re located on curves. The overrunning third rail would have to be converted to underrunning. They also use different voltages, although off the top of my head I can’t say how difficult it would be to reconcile them.
• The El operates with multi-car trains and station-based fare collection. The NHSL uses only single or double cars and onboard collection. Stations would require full gating à la Millbourne. Ridership is about 10K per day which probably wouldn’t justify running six-car consists beyond Upper Darby, so either El service would have to be reworked to use shorter trains or have a mixture of six- and two-car consists. That’s certainly not impossible but could create boarding issues for the much larger passenger loads typical on the El.
Bottom line, the gain probably couldn’t justify the cost.
It’s hugely unfortunate that integration of this area's transit system is hobbled by the legacy of incompatible standards either created by uncoordinated private developers or forced on it by political decisions (most obviously "trolley gauge"). Given enough money, my humble (?!) proposals for physical integration/reconfiguring the Red Arrow / PTC interface would include:
- Redesign 69th Street to bring the four rail lines into a common station that also supported as many bus routes as possible, with less walking and better weather protection.
- Extend the NHSL east to meet the proposed PATCO extension at University City. Although operational differences are about as great as with the El, the two systems are at least physically compatible. That opens up the possibility of some service overlap if not necessarily full through-running. FWIW, something similar to this was first proposed by the DVRPC nearly half a century ago.
But I dream ...
Requiem for it's/its, your/you're, than/then, less/fewer. They were once such nice words with such different meanings...