The only thing that will drive LUV back on PIT-PHL is if the high speed train opening next month from PHL-MDT is extended to PIT. US won't do it.
HA! That's hardly "high speed". The new max speed on that route is 110mph. The rail industry definition of a high speed train is 200 km/hr, or 124mph.
Amtrak will be using the same 30 year old equipment that's been running on the Northeast Corridor. These trainsets are capable of 125mph, as they have run for many years between DC and NYC under the Metroliner service (now retired) and, more recently, the Regional service. So, PA is basically getting hand me down equipment on 100+ year old tracks that have been given some TLC to accomodate them.
PHL to Harrisburg will not likely ever see the Acela which is
capable of 160mph, but rarely goes over 135mph on it's entire NEC run (except a very short stretch in Rhode Island)
The PA Govt. heavily subsidized Amtrak to fully electrify the PHL-HAR route and upgrade the signaling and catenary to allow faster speeds because there's a large contingent of people travelling along that line, which has also shown heavy growth over the years. It's relatively straight and open, too, so achieving the 110mph speeds will be completely doable. And, 90 minutes between HAR and PHL is very attractive if you've ever driven that route and gotten stuck on the dreaded "SURE-kill" expressway .
By comparison, the tracks running from HAR to PIT are windy and run through many mountain passes (including the famous Horseshoe Curve in Altoona). Amtrak will never do it. A private consortium will never do it. The freight lines will never accomodate anyone anyway. And, even with the best possible speeds given the condition of the line, a trip from PHL to PIT - approx. 300 miles - would still take well over 4 hours.
I think I read somewhere long ago that PIT-PHL and LONDON-PARIS were two of the world's most heavily travelled short-hop routes (however they defined those). This may have actually been prior to the success of the Channel Tunnel, though.