Otto nailed it.
To this day, as you ride the Cascades or the Coast Starlight north of the Toutle River, you'll see huge mounds of gray ash that was piled up after the May 18, 1980 eruption of Mt. St. Helens. It's all still there (and sadly, there are many places that'll CHARGE you for authentic Mt. St. Helens eruption ash, when you can dig it up yourself for free.) Of course today it's more a favorite for quad riders to ride up and down, right next to the busy mainline...
In the event of an eruption, you would have a large plume of ash, plus snowmelt and glaciers melting rapidly causing flash flooding. Much of this ash and floodflow will go west, right into the path of the BNSF mainline (and I-5, for that matter). Just like in 1980 - EVERYTHING will shut down. No planes, no trains, no cars or trucks.
Amtrak resources? Best thing Amtrak would do is tie everything together, and get the equipment out of the way - IF there was warning. Of course, hundreds of evacuated residents thought that they'd be able to go in the morning of May 18th to gather the last of their belongings. And Harry S. Truman looked up, probably uttered some words not appropriate for this forum, and gasped his last breath of air.
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Erik Halstead - Portland, Oregon