The question comes up on here with some regularity about boarding outside of the main concourse via NJ transit or lower levels.

I believe only 2 platforms that serve the west end concourse serve Amtrak trains. The rest are exclusively LIRR. So it's a pretty good roll of the dice unless you know you're trains boarding from one of those. Also, if you get sent back it's a long hike to the central concourse.

I've done it at least once mainly by accident coming off the C subway. I just remember a really long walk on the platform since the train was parked to be closer to the intended boarding area.