Normally when we discuss a direct PHL-CHI train, the most common solution is through cars connecting the CL to the Pennsylvanian, according to the PRIIA from 2010:
https://www.amtrak.com/ccurl/152/943/PRIIA-210-CapitolLimited-PIP.pdf
To me, the key criteria are the travel time between the two cities and whether or not a dreaded transfer is required.
Current options:
43-29: 12:42pm - 8:45am next day (Layover 8:05-11:59pm in PGH)
95/195-29: 12:02pm/12:34pm - 8:45am next day (Layover 2:00/2:25pm-4:05pm in WAS)
84/88-49: 1:05/1:19pm - 9:45am next day (Layover 2:30/2:46-3:40pm in NYP)
51: 8:15am-10:05am next day
If the PRIIA plan is implemented, the train will leave PHL at the same time as 43 but 29 will arrive in CHI at 9:05am (20 min later). So that would add 20 min to the trip although you can stay on the train during the 4 hour layover (29 would leave PGH at 12:20am instead of 11:59am so the layover would exceed 4 hours).
Comparing to some of the old trains:
BL, April 1990 (via Ft. Wayne/Valparaiso): 4:11pm-8:27am (
http://www.timetables.org/full.php?group=19900401b&item=0044)
BL, October 1994 (via Garrett/Nappanee): 2:57pm-7:25am (
http://www.timetables.org/full.php?group=19941030n&item=0018)
TR, May 2000: 3:00pm-8:25am (
http://www.timetables.org/full.php?group=20000521n&item=0030)
Compared to 2000, today's trip takes about 3 hrs longer and requires a transfer. The trip was about an hour shorter in 1990/1994 than in 2000 so compared to the 1990's the trip takes about 4 hours longer today.
1994 trip: 17 hrs, 28 min (1048 min)
2016 trip (43-29): 21 hrs, 3 min (1263 min, about 20% longer than 1994)
2016 trip (51): 26 hrs, 50 min (1610 min, more than 50% longer than 1994)
I would say a reasonable expectation/goal for PHL-CHI should be 19 hours (2000 TR was 18 hrs, 25 min).
Another consideration should be the train should serve as a 2nd NYP-CHI train to provide relief to the LSL.
Current 43-29 trip from NYP-CHI (change trains in PGH): 10:52am-8:45am (22:53)
Current 95/195-29 trip (change trains in WAS): 10:35am/11:05am-8:45am
Current 49 trip: 3:40pm-9:45am (19:05)
Transferring to the CL adds about 4 hours to the trip. Even if no transfer was required (PRIIA proposal), that would still be considerably longer than the LSL. By contrast, the 1994 trip on the BL was 12:45pm-7:55am, only about 1 hour longer than the LSL today so it would be more competitive with the LSL. If the goal from PHL-CHI is 19 hrs, then the goal from NYP-CHI should be 21 hrs or 2 hrs longer than the LSL.
At least considering westbound, realistically if the layover in PGH is cut to 2 hours as opposed to 4, you'd get the desired 19 hr trip between PHL-CHI. You could either have the 43 arrive in PGH later and/or have the 29 leave PGH earlier (and this would make travel from PGH-CHI more desirable since you'd leave PGH earlier). Hopefully if PennDOT/NS increases the frequency between NYP/PHL-PGH they could move the 43 back closer to the 29 whether or not the through cars are added (of course adding them would be preferred). The eastbound layover between the 30-42 is 2:25 (5:05am-7:30am) but no one wants to leave a train at 5:05am if the transfer is required (or for passengers leaving at PGH it doesn't matter). Ideally the CL would change their schedule (earlier arrival into/later departure from CHI) but the Pennsylvanian changing its schedule also would improve the trip.
I'm hoping if they add frequency along the Pennsylvanian route it will eventually improve the PHL-CHI (or HAR-CHI, Lancaster-CHI, Altoona-CHI, etc) trip as well.