Right now on Capitol Corridor they're mostly carrying portable printers that generate a printed seat check. It's complete with date of travel, the origin station, destination station, train number, and reservation number. Roadman took a photo of two of them printed back to back for two passengers:
Eventually this is going to be rolled out completely on Amtrak, or so I've been told. At that point I guess the codes that conductors use will go away. I would think they would also have the ability to print the car number and seat number for those on long distance trains with assigned seating.
Yeah, I too have heard that the plan is for this being rolled out system-wide. The system is pretty darn proven, and not just used with Amtrak. A full-service carwash here in Sacramento uses what looks to be the same exact system. An iPhone is used to take the order, or call up an account, and a belt-mounted printer identical to Amtrak's produces a barcode receipt you take up to the cashier.
In the case of Amtrak, you'll note that it contains a lot more data than just the station 3-letter code. It has the res number, and a barcode. As such, a conductor can (and I've witnessed them do it) walk down the isle and scan a suspicious tag to see if its valid. And in the case of the person involved, it wasn't (they'd kept their seat tag from an earlier ride and tried to reuse it.) That person had to either pay the fare to the conductor right there, or be seen off the train at the next stop and into law enforcement's hands. They paid up.