From WAS to NYP, trains 2124 and 2126 (6 and 7 pm Acela) have been full on Thursdays for several weeks running. I've arrived early for 2124 and found I had to take 2126, or arrived early for 2126 and found I had to take a regional, several times. I hope Amtrak opens an additional Acela service or adds a car to an existing service.
An additional train might be possible if Amtrak can clean them fast and get them turned around fast enough.
It is not possible however to add a car to an existing Acela Express train. First, there are no extra cars. So Amtrak would have to shorten one Acela train to make another longer. Second, unlike normal trains, to add or remove a car requires that the entire train go to the shop and spend several hours in the shop. Acela doesn't use traditional couplers, so cars can't be easily added or subtracted.