I really don't know, but I don't think you can change a name of the passenger on the reservation. You may have to cancel the original res and make a new res. However, that may result in a higher fare and/or loss of any discounts.

But there may be a way - if you want to gamble! Amtrak does random (and the key word is random) ID checks. (It is so random that I was never ID'ed - although others have been many times.) If the reservation was for Bob or Mark and you are a Ed or David, you may not get questioned. But I don't think it would be as easy if you were a Sue or Mary!

Now I'm not saying you should, but I'm just saying it may be possible. I'll probably get ID'ed the first time I try it, or on my next trip.