At Penn station, you used to be able to wait on the floor below (which has a staircase from the main floor as well as its own staircases down to each platform) and then just directly go down to your Amtrak as soon as you wanted rather than waiting for them to form a giant line. My dad taught me this trick, and we both used to use it to be able to avoid waiting longer, but once when I did it one of the Amtrak workers got super angry at me for it despite the fact that I did wait until after everyone leaving had come up, and as a socially awkward teen, I was too nervous to ever do it again.

I suspect the rationale behind not telling people beforehand is that they don't want to have to deal with people trying to get on at the same time as people leaving, but I feel like the main reason people are so aggressive about trying to get to the train as fast as possible on Amtrak is precisely _because_ the system treats them as too irresponsible to get timely accurate information. If people didn't need to worry about having only a couple short minutes to avoid missing their train and could plan ahead where to be when the train arrived, I think that a lot of people wouldn't feel the need to try to jostle through the crowd to race each other for the earlier spots in the line.