For the geeks out there, I ran another barcode that appears on an e-ticket printed by a Quik-Trak kiosk through a barcode decoder, and have one correction to my previous post. I also discovered one quirk, as you'll see below.

The only data that the bar code contains is the reservation number and date of issue (not travel, as I originally thought). However, the format of the reservation number differs depending on the barcode format.

The QR code that appears on the e-ticket PDF that Amtrak emails you or that appears on the mobile app, the data embedded in the code is formatted "123ABC-11FEB13".

However, the PDF417 barcode appearing on the old ticket stock has the same data embedded as "0119468420130211", where 01194684 is the reservation number and 20130211 is the date of issue (in this case the 11th of February, 2013).

The computer geeks out there may be quick to notice that these reservation numbers are actually the same--the only difference being that the QR code has it expressed in hexadecimal (and the way it would be read to you over the phone and printed on your itinerary), while the PDF417 barcode printed on old ticket stock is the decimal version of the number.

Why do they do it this way? Only Amtrak IT would know--after all, alpha characters can be embedded just as easily into a PDF417 formatted barcode as into a QR code.

However, it does explain why all Amtrak reservation numbers only use numbers plus letters A through F, unlike airline PNRs, which make use of numbers plus all letters (with the exception of easily confused letters like I and O). I highly suspect that the database behind ARROW is set up so that a reservation number uses exactly three bytes of memory (each hexadecimal number requires four bits, or half a byte)--to use a full alphanumeric scheme like an airline PNR would traditionally require six bytes of memory. And while the difference may be trivial now, when the computer reservation systems were first implemented decades ago a difference of three bytes (both for data storage and data communication purposes) would have represented significant savings and efficiency.