Another factoid which might influence you: Amtrak used to have a very generous cancellation and refund policy. Essentially, as long as you canceled before the train departed the station, you would get at least full credit towards another trip. Well, of course, people started abusing that. Some tour agencies were notorious for booking every sleeper accommodation on popular trains such as the California Zephyr to "hold them for ransom"...resell them to customers at a substantial markup, who couldn't buy from Amtrak because the train showed as "Sold Out"...and then, if any didn't sell, cancel them at literally the last minute for a voucher which they could use to do the same thing all over again next year. Ordinary travelers were essentially frozen out of the popular dates.
So now there's penalties if you downgrade or cancel a sleeping accommodation. However, it's not a total penalty...you still get something like a 60% credit. So (just pulling numbers out of the air), if you purchase a sleeper upgrade for $550 (total) but at some point before departure the fare drops to $450 (and you find out about it!), you can call and have an agent modify your reservation for the lower fare of $450. You'll pay the 40% penalty on the downgrade, but your net cost will be $490 when you expected to pay $550. I know you can get the $60 difference back in a voucher but I need to review the rules; depending on when you modify you may be able to get the difference back in actual cash (to your credit card, of course).
And, if the price doesn't drop, you've still got your original $550 ticket for a private roomette with meals. Just consider it as buying an option put to hedge your bet against a price drop....