Only today's pre-teeners might see superliners on the NEC. That of course if SL-5s have a high level boarding area at one end, east river tunnels 5 &6 built, Harold underpass built to SL clearances, Penn south built, Gateway tunnels & other projects to Newark Penn, Newark Penn clearances fixed, 30 St clearances fixed, BAL clearances and B&P tunnels built and any other clearances, WASH US clearances.. Also probably 60 Hz CAT. Not much ? Final kicker -- the present North river tunnels can never be used for SLs
Easier will be lengthening trains first. The present layout of BOS will not allow the present 9 car length to expand beyond 11 cars so that will be a limit. As well winter servicing bays for that length will need to find a spot to be built. But NYP - WASH can get longer lengths without much work. ( relative ) to meet an 18 car goal ( ACS-64s planned capacity ). That car need of course will require many V-2 or V-3 coaches to be built to meet capacity needs.