Hi Lou -- Sorry that this is coming right in the time that you were planning to transit there.
Does the Eurostar ticket go only to Brussels, and then you change trains to go to Amsterdam? Or is the Eurostar all the way to Amsterdam.
The Eurostar is the ONLY train in Europe (that I know of) that does in fact have airline-type security that you have to pass through before being allowed to board, so if I was taking that train somewhere, I would feel reassured and I wouldn't change my plans. That's MY appetite for risk though and may not match yours.
if you have to change at Brussels, one thinks again that surely it would be okay, but again we can never know.
I don't know that you'll have any luck getting Eurostar (and/or the other train company if there is one) granting you a refund. I unfortunately don't have any brilliant ideas for trying to make that happen. All I can think of is that maybe if they end up having to cancel trains due to security concerns, then they'll have to refund everybody.
I'm sorry not to be able to offer better advice. Good luck to you as you try to figure out your next steps.