So, having dealt with a train cancellation before (found out when we arrived at the train station in BC), I will tell you that it is NOT lkely to be a normal Amtrak bus. They have those scheduled already. So a person dealing with a cancelled train might, if the same thing happens with them as it did for us, get a chartered, unmarked, non-Amtrak train.
They know that people have the bags with them that they expected to get on the train. While a normal Amtrak bus might have limits, the ones chartered and put into service for people who booked a train will be different. They have to be.
This isn't something I would worry about.
Oh and for those that book business class, IF your train is cancelled and you're on the bus, you can expect to get the business class upcharge refunded. Others will get nothing refunded.
http://www.amtrakcascades.com/Baggage.htm
Sharksfinatic, you know that the normal baggage allowance for the train is 2 bags, and the medical equipment allowance is listed under carryon?
And everyone remember that BNSF is freight, and freight owns the rails, and the rails carry a ton of stuff north and south. They aren't going to close the rails for an extended period of time unless something drastic happened.