A US News and World Report article indicates airport security have carte blanche to search passengers to unreasonable extents.
This FlyerTalk forum gives further details, and they're not pretty. The best tip from there is to wear thin clothing which would not easily hide dangerous devices beneath, and thin sandals, not thick-soled footwear.
And, it appears there's something going on at PDX which points to managerial problems with the TSA screeners.
So far, Amtrak is only checking IDs under their new security plans, and leaves the groping to the TSA.