Amtrak's boarding procedures come straight out of NJ Gov. Chris Christie's "Time for some traffic problems at Fort Lee" transportation planning department.
There is absolutely no inherent reason to make people line up or wait before boarding a train.
Just open the doors to the tracks when the train is ready for boarding and let people board. Period. That's how NJ Transit, the Long Island Rail Road, Metro-North and plenty of railroads around the world do boarding.