TRRed : The Amtrak Guest Rewards Mastercard (issued by MBNA) sounds like an ideal fit for you,. No annual fee, one point per $, and points can be converted into CO, YX, or UA miles 1:1, in increments of 5,000. The biggest drawback is the limit of 25,000 miles per year for transfers out of Amtrak, but as this will be a secondary card for you, that might not be important.

The normal signup bonus is only a measly 500 points, but there's a 2500 point offer floating around there, which may or may not be targeted. See http://www.flyertalk.com/forum/showthread.php?t=327026