Also, if anyone would like to consider the parallels between the Metroliners and the Acelas...both winding up doing post-prime duty to Harrisburg (the "Capitoliners", if I'm not mistaken).
Edit: To be fair, I'm not opposed to using them on the Harrisburg run...that would at least free up a bunch of Amfleets for NEC service. However, I also don't think there are enough Keystones to optimally use the Acelas.
The 6-car sets are pretty much the optimal *length* to handle the Keystone platforms (yes, I checked information about platform length and trainset length -- the Acela trainsets are longer than the shorter 550 ft. Keystone platforms, with the passenger cars just about fitting on the platforms). Too few seats is an interesting point, but seems to be solely due to the Business Class/First Class seat layout and can probably just be changed by adjusting seat pitch.
I'm not thinking of optimal usage exactly -- I'm thinking of what to do in the short run, when the Metroliner cab cars start to crap out, and no new cab cars have been ordered. There seem to be 16 or 17 Metroliners (depending on whether the most recent wreck gets repaired), and they're ancient with few spare parts. If you scrapped several of the Acelas for parts you'd still have more than you have Metroliners.
The alternative is converting some locomotives to cab cars, of course, but that's (a) more expensive, (b) lugging more dead weight around, © requires maintenance of another class of obsolete parts (either for HHP-8 or AEM-7), and (d) continues to tie up the Amfleets, which are always in demand elsewhere.
Exton high platforms are done. Look at Amtrak's website; Exton has high platforms. The rest of the station is unfinished but that's not important for this purpose.
Paoli Transportation Center was put off to the FY2020-25 timeframe, but that could be a placeholder while SEPTA, PennDOT, and Amtrak complete the EIS and locate the funding.
Amtrak committed to build the platforms in advance of the rest of the station, in a lawsuit settlement with the Disability Rights Network of Pennsylvania. If you read through the details, the high platforms have to be done by 2018.
http://disabilityrightsgalaxy.com/settlement-reached-to-improve-amtrak-station-accessibility/
Regardless of the status of high level platforms for the stations on the eastern Keystone, for most of the reasons expressed by other posters, I doubt that the Acela Is will be repurposed for use as Keystone trains. PennDOT won't want to pay more,
Why would it pay more? They'd be paying to use used trainsets which aren't in high demand, and which would otherwise be mothballed. The alternative is paying for in-high-demand ACS-64s and Amfleets... PennDOT might appreciate the higher acceleration and the tilting, though.
too expensive to operate,
Operational expense is a real question, but I don't know the actual numbers. The Acela coaches are significantly overweight compared to the Amfleets, so that is expensive, and that may be a good enough reason to get rid of them. But the power cars are lighter-weight than the ACS-64 and comparable to an AEM-7, and it appears that the Acelas can accelerate faster. So I think they're going to be preferable. Diesel fuel costs are a small but significant fraction of Amtrak operating costs; electric fuel costs are a small and insignificant fraction of Amtrak operating costs.
The power supply question is more critical, but those power stations are being upgraded anyway... and they wouldn't be running at 150 mph.
too few seats in the current 6 car configuration, NYP maintenance facility not able to handle extended Acela consists, BC seats, what to do with the first class car, unused bistro car, and so on.
Reconfigure them as 10 6-car trainsets of all coach, if you like.
By 2020 or 2021, the Acela Is will have served their purpose, introducing HSR style trainsets to the US and having been quite successful in growing ridership on the NEC. The Acela Is are likely to get sent to storage and kept there until Amtrak disposes of them.
These aren't Turboliners -- they're much more efficient to operate than that, and there are more than enough spare parts.
As for aging Metroliner cab cars, Amtrak can convert some AEM-7s or get a set of Viewliner II cab cars in the first batch of a single level corridor car order.
I'm absolutely sure that Viewliner II profile cab cars are Amtrak's future plan, but they certainly won't have arrived by 2018, and probably not by 2021.
Converting AEM-7s, which date from 1979-1988, to cab cars is frankly more questionable than repurposing the Acelas for a few years. AEM-7 cab car conversions would require substantial work. Even if AEM-7s are converted to cab cars, they'll probably immediately be in demand on various diesel-hauled routes.
As I've said before, the other option of using them in some sort of Regional service would make sense; moreover, I feel compelled to point out that if the Acelas are used on the Keystone route, you're going to end up in a perverse situation where the "commuter" trains connecting to Harrisburg have a higher speed limit on them than do the Regionals. That's going to create some...interesting fights if you've got Keystones able to beat Regionals between NYP and PHL (something that won't be helped by the NJ speed upgrades).*
Easy to deal with: price the Keystones higher than the Regionals (and lower than the new Acelas) between NYP and PHL. The market is big enough for that level of segmentation, and Amtrak needs to try to keep local traffic off the Keystone from NYP to PHL anyway to make room for PHL-HAR traffic.