Originally Posted by
palffy
Xyzzy, I'm going the other way

(flying out of EWR). So ABE-ZFV-EWR-XXX (ABE-EWR is the same Northeast Regional train) then XXX-PHL-Septa-ABE a week later..
After you check in online with UA, call Amtrak and get your ticket number. And at ABE have them issue the ticket there. Use your own purchase till PHL (ZFV) then use the UA/Amtrak codeshare ticket to EWR.. Should work, no?