All railroads are required to perform operations testing by the FRA. One of the railroads' favorite tests is to see if the crew is complying with Restricted Speed by waiting for them downline to see if they are in compliance.

They will place "STOP" banners across the tracks, or light a fusee, things like that to see if the crew can stop in time. Usually, the trainmaster or road foreman will come up in the cab by themselves, but sometimes they are lazy, meaning the conductor/engineer has to climb down.

I wonder if the Amtrak crew got off the train because they mistakenly believed the kids were UP officials? Of course this is all speculation, but I can definitely envision a scenario like this happening, especially at night, when you can't get a good look at someone until you're up close and personal.