I think it would increase ridership if some of the trains that terminate at/originate from PHL would do so from Suburban Station. This way, commuters would be able to get a one-seat ride directly into the heart of Center City, which might persuade even more people to ride the train. The only concern would be the bottleneck at 30th Street, since they'd have to use the upper level, which has only 2 tracks per direction of travel. Once the train gets past 30th, it could lay over on Track 0, which is almost never used.
The upper level of 30th Street Station has 6 tracks, 3 in each direction. The only "Philly" station with just 4 tracks is Market East, which would not be a good place at all to try and turn an Amtrak train.
The problem with running to Suburban is that you'd either have to run a diesel into that station, which SEPTA probably would not want or Amtrak would have to change to an electric engine at Harrisburg and that would slow down running times.
What are the rules on running a diesel into Suburban Station? Academic question, though, because I don't see Amtrak going to the operational trouble of moving a train into Suburban Station if it is the only daily train to do so. The PHL-PGH train would then have to use the upper track platforms at 30th Street north of the main hall, which would confuse people who are used to only getting on Amtrak at the lower platforms.
No, a PHL-PGH train should stick to departing from the lower platforms. With no need to reverse the train direction at PHL, it would not need a cab car. Another cost saving item for a direct PHL-PGH train.
Trimming the trip time for a PHL-PGH round trip train where possible will improve the round trip scheduling and increase ridership. What is needed, beyond the funded HSIPR projects, for 15-20 minute trip time reductions on the electrified Keystone East was summarized in the PA DOT applications for the FL HSR funds. Another $200 million or so beyond what was granted for the Keystone East along with station upgrades.
Checking the Planthekeystone website, I see the Keystone Corridor West Feasibility Study was expected to be completed in winter 2011-2012. Well, since it now officially the spring of 2012, they missed their projected release date. If the study contractor has not slipped too much, we should have a feasibility study report for HAR-PGH improvements and rough cost estimates to mull over soon.