In the remnants of the formerly great transfer opportunity (YX or CO) - Amtrak - Choice - Any Airline, there's been some mention of a recent increased tightening of rules for the transfer out of Amtrak. I haven't seen this mentioned, from the Midwest Miles Monthly:
Transfer Fee. As of July 1, there will be a $25 service fee on all mileage transfers out of your Midwest Miles account to Hilton or Amtrak.