That said, Amtrak engines are also often leased to other freight companies so you can frequently see freight trains being hauled by Amtrak branded engines.
http://utahrails.net/pass/amtrak-mail-express.htm
It stopped in 2004, presumably due to declines in mail volume. (Which I wonder if it would make sense again now, due to more mail-order package shipping.)
Anyway, it's not really "freight" in the sense that most people think of a freight train. It's all Amtrak rolling stock.
They still do offer Express shipping services between some stations. It's a very cheap way to send something between NYC and Washington for same-day arrival, especially if it's a big box. You have to have someone go to the train station on either end to get your stuff, though. They move them around on the passenger trains that have baggage cars (the 'named' trains, typically at least on the East Coast).