This may sound like a dumb question, but why doesn't Amtrak just lease some different engines from BN/SF or other railroads to fill the power void?
Some things right off the bat:
Freight locos usually cannot provide proper head-end power for the train.
It costs a lot of money, as not only are you renting power, but a crew as well to run the freight locos.
Most freight railroads do not have the resources to lease power AND crews for passenger service on a regular scheduled basis. You are taking away resources they could be using for their regular customers.
Freight companies are not excited at all about having their equipment and crews involved in operating a passenger train. Their liability and chance of a lawsuit increase by a factor of about 500 or more, should something happen while they are pulling the passenger train.