<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Originally posted by gleff:
Amtrak transfers allow you to move pretty much any airline into United miles.
AA: credit your miles to Midwest and transfer Midwest --> Amtrak --> United
NW: credit your miles to CO and transfer --> Amtrak --> United.
DL: credit your miles to CO and transfer --> Amtrak --> United.
AS: credit your miles to CO and transfer --> Amtrak --> United.
US: credit your miles to UA. (obvious)
UA: credit your miles to UA. (obvious)
YX: transfer your miles --> Amtrak --> United.</font>
Another angle:
Amtrak points can be used for flights on most US airlines and many interntional.
GR = Guest Rewards
GR --> CO = DL, NW, KLM, et. al.
GR --> YX = AA, F9
GR --> UA = US, *A
Also, GR can be turned into HHonors points:
GR --> HH 1:2
Or redeemed directly for Marriot and Starwood. The Starwood redemption scheme is not favorable, but the Marriot one is decent, IMHO.