yup, it stands for Canadian Border.
Don't make the mistake I did back in October. Amtrak printed (and I believe still does) two tickets on ticket stock for the NYP to Toronto trip (NYP to CBN and CBN to Toronto). I mistook the CBN to Toronto ticket for my receipt and left it at home. The VIA crew which reboarded the train after clearing customs was not amused or sympathetic -- it cost me an additional $55 bucks on board to get to Toronto.
Other than that it was a great trip.