So, I've been poking at this and really the question is whether Amtrak is looking to expand the offerings here, potentially add a stop or two at each end for the "prevailing" direction, etc. Prevailing air fares on the DCA-LGA route, same-day round-trip, are $971 in the next seven days, $701 at 8-14 days, $551 at 15-21 days, and $331 from then onwards. Going to a one-or-two night stay does some "damage control", particularly starting at two weeks out (and with two airlines running the route there's room to do some "nesting" across AA vs DL), but the close-in Y fares are still far above the Acela. Pricing is a bit better at JFK, but frequencies are also thinner (and for reasons known only to God, folks from New York apparently prefer LGA to JFK).
As such, if they can get to the point that they can maintain a 2:30 timetable (+/-5 minutes), possibly with a sole intermediate stop (BAL would be my first choice as long as it remains in a slow patch) I would be not at all surprised if Amtrak could actually make a serious go of finally starting to drive the air shuttles under (they've already seriously dented them, to the point that AA abandoned hourly service while both AA and DL are using Embraers instead of 737s or A320-family planes; I honestly don't think the air shuttles can take another hit on that scale).
By the way, I'm going to pretend I'm Richard Anderson for a moment (which is a horrifying thought). If I'm in his shoes, I might actually chose to run the expresses at, if not a loss, then at a very marginal direct operating profit in a serious attempt to damage the air shuttles badly enough to try and force a frequency collapse or knock one of the airlines out of the shuttle part of that market (as opposed to with connecting traffic...which really makes more sense to run through airports without the relevant congestion problems). At that point I can turn around and try to jack my fares. It is a brutally predatory move, it is an aggressive gambit, but I think it has room to succeed (particularly, as indicated, if BAL/BWI were to be thrown in without having to add time to the run...but at least in the short term I think part of the pitch will be "look at the shiny nonstop").