Once upon a time long long ago in a place far far away before Amtrak, NJTransit, Septa, Marc, etc, there was the Pennsylvania Railroad. In those days services ran under the railroad’s name, unlike NJT’s i.e. Northeast Corridor Line. Thus before, was Pennsy Trenton NY local; express, Trenton Phila local; express, NY Wash etc. Ironically I believe there were more choices of services to choose from by which I mean more mixed services. i.e. NY Phila Local. This means commuter services running a straight one seat ride from NYC to Philadelphia stopping at stations, not too many, like Rahway NJ which now Amtrak does not stop, Morrisville PA etc. This is equivalent to a modern day NJT Jersey Arrow running a service straight from NY to Phila with some intermediate stops unlike Amtrak, or a modern day Septa Silverliner running straight from Phila to NYC with some intermediate stops unlike Amtrak. Nowadays if you want to go to Phila from NY you take NJT to Trenton then change to Septa, or you take Amtrak, thus it’s too expensive when reachable by commuter rail. Thus the Northeast Corridor is divided into regions. Legend: “to” means divider (division) i.e. Boston “to” Rhode Island “to” New Haven “to” NYC “to” Phila “to” Wilmington “to” Baltimore “to” Wash. A typical commuter rail line should be an average of 25-30 miles from work, but my experience here in the Northeast told me it’s more. NY to Phila commuter service was accepted in the old days. This is a 91 mile commuter trip. A longer distance will be unreliable thus possible i.e. Boston to Wash local commuter service with all stops!!! Possible but screw that, and with traffic management, more trains, and how long that would take, forget that!!! That’s why the Northeast Corridor is divided into acceptable regions like NY to Phila can handle passenger and commuter services. Back to square one. I wonder if at one time trains ran like a straight one
The New York-Philadelphia trains to which you refer were numbered in the 200 series on PRR and were informally referred to as “Clockers”, although this name never appeared in any timetables. They were short-haul intercity trains that happened to handle some commuters during peak periods. More stops were made than NY-Washington trains or the east-west long-haul runs but less stops than corresponding NY-Trenton or Trenton-Philadelphia suburban locals.
In response to other questions, the Princeton branch only operated as a shuttle for scheduled services, some football specials and other unscheduled trains did run through to New York or Philadelphia. All Philadelphia suburban locals terminated in Philadelphia, no through runs to elsewhere.
Doubt there were ever any locals from Paoli or Chestnut Hill all the way to NY. The last Phila-NY local ended around 1959, and there weren’t more than a couple a day even 20-30 years before that.