That is dangerous in NYC. Penn/Moynihan Station is a terminating station. I'm guessing over 60% of the train capacity often unloads at Penn/Moynihan Station (assuming NE corridor from DC with stops in philly and other towns along the way). You can't have passengers waiting to board obstruct that many people disembarking on a narrow platform. You're thinking of suburban train stations that just quickly pass through, not a major urban station. Also, because it's amtrak there's a lot of luggage being carried which needs more space. Not to mention room for dedicated luggage service to move bags to baggage claim.