You could do the Chase Freedom now, and maybe somewhere down the road add a Chase Sapphire Preferred. Chase Freedom doesn't have as generous as a signup bonus, but has rotating 5x categories that can be converted to UlimateRewards points (with the addition of an UltimateRewards card like Chase Sapphire Preferred.) Add a CSP and then you can transfer your UltimateRewards points to Amtrak.
Right now the CSP is offering 40,000 after meeting minimum spend. Who knows what it will be six months or a year from now, but I think the highest publically available offer for the Amtrak Guest Rewards card is currently 12,000 points. I have seen it for higher, but nowhere near 40,000 -- plus the Amtrak card only offers 2x for Amtrak purchases while the CSP offers 2x on any travel, access to more earnings through the Ultimate Rewards Mall, and the flexibility to use the points for more than Amtrak (cash rebates or transfer to hotel and airline partners.)