That size won't fit in the overhead bin. Large sizes would go in the racks at the end of the car.
And in certain cars (both end cars) there is the old phone booth that has been removed, and we typically store larger bags there, or behind two sets of seats that have seatbacks facing each other (that's only really applicable in the First Class car however.
Also, I'm just picking apart the diagram of the First Class car, so I'm not sure how accurate it is. There are two sets of single seats on both the left and right side that are facing each other and have a table in between. There is no gap between the ADA seat on the bottom right of the car (the wall is right behind it, with luggage storage there), and the smaller bathroom does not exist, as that is the defunct phonebooth.
Also, have you looked at this? https://www.amtrak.com/media/acela/acelaTour.html In particular, under "other amenities"... "overhead bins".
