On the 8th Ave side, take the steps or escalator one level down to the subway level and wait for your Acela. You will see departure screens between tracks 12 and 1 that will notify you of which track is your train when it is ready for boarding. Then you can quickly go to your track.

If you were wait upstairs in the main waiting area, you would probably wait on a long line to descend the steps or escalator to your track. Go where the commuters wait which is the subway level to avoid that hassle.