I have never figured out the "signing tickets" policy. I once heard that it was to get a lower transaction fee for credit card purchases, but that is not accurate. The only way a signature would matter to a credit card bank is if it were the cardholder's signature at time of purchase. The ticket is signed by the passenger, who may or may not be the cardholder, and it can be signed weeks or months after the purchase. Is it to ensure the ID of the person surrendering the ticket? Doubtful, since 99% of the time the signed ticket is lifted without the conductor even asking for ID. My opinion: this is one of those things Amtrak has always done and they still do it even though there is no reason for it.
It's all of the above. It 'prevents' ID theft, it may get them lower rates, it makes a contract between the 'cardholder' and such...
The policy is indeed because credit card companies offer a lower fee per transaction where the company processing the charge obtains a signature.
The problem lies in the fact that like many things, Amtrak does a poor job of communicating to their employees why the tickets need to be signed and just who should be signing them. One can't ask for id to confirm that one is indeed the card holder according to credit card company policies. That is illegal in fact I believe, which is why Amtrak doesn't ask for ID when signing a ticket.
On the other hand, Amtrak should indeed be making sure that the card holder is the one signing the ticket. Making someone else sign the ticket in effect could allow the traveler to disput the charge. If I buy the tickets, then I sign the tickets. If my mom brought the tickets, then she signs all the tickets. That is the way it's supposed to be. But again, most Amtrak employees don't understand why people have to sign tickets, much less that it must be the credit card holder.
You do not sign AGR tickets, tickets brought with cash, or with debit cards, or via an Amtrak voucher. I've been asked to sign those tickets, and I refuse everytime. Even had a sleeping car attendant on the Eagle a few years back who wanted me to sign my AGR issued ticket. I tried to politely explain to her why I would not sign the ticket, but she would have none of that. She then went into the dining car and I could see her telling the conductor that I wouldn't sign my ticket.
He came by my room a few minutes later and asked me what the problem was. I told him that I had no problem, just that she wouldn't take my ticket from me because it was unsigned and since it was an AGR ticket it didn't need to be signed. He looked at my ticket for a second, saw the zero dollar amount and lifted my ticket without further comment. I could see her face in the dining car and I could tell that she wasn't happy that he wasn't yelling at me.
I did finally manage to explain to her the following day why I wouldn't sign it and why the conductor agreed with me.
Signing tickets does not prevent ID theft, if I've got a forged ID, then the signature is still mine. Signing a ticket only makes a contract if indeed the person signing the ticket is the card holder or an authorized signature on that account. Otherwise it is just so much wasted ink. If my wife signs my ticket, depending on which card I used, it would be a contract. If my mom signs a ticket that I purchased using one of my credit cards, then Amtrak has nothing. If I were to dispute the charge, I would win and Amtrak would loose.
So all that said, I never sign my tickets until minutes before I board my train and all tickets are signed by the person whose credit card was used to purchase the tickets. And again, I never sign tickets from AGR or tickets that were issued by voucher or paid for with cash.