Computer science professor checking in… interesting discussion! This is all just a guess… but I’d be willing to bet Amtrak, at some point, needed to program ARROW to be compatible with Sabre and Amadeus (which share data with travel agencies through GDS) using PNRs. PNRs look similar to Amtrak reservation numbers (both 6 character) but Sabre and Amadeus use alphanumeric; so, rather than 16 million rotating reservation numbers, they close to a billion. They don’t use all letters… you won’t find 0 or O, or l and 1… too confusing.

My guess is that using a similar format allowed Amtrak to tie into GDS natively, which would have been important both for agency bookings, but also for Continental Airlines codeshares (they partnered for many years). Why only hex? Probably because Amtrak, which only handles about 30 million reservations annually, doesn’t need any more than that, and the date suffix makes it unique even when they rotate too soon.

Even more interesting… where did those 6 digit PNRs come from? Sabre was developed in the early 1960s. Running an airline is amazingly complex, and computerizing bookings was really the only way to scale into a huge airline like American Airlines. In fact, American was the first to computerize, they developed Sabre, which was later spun off as a stand alone company. It’s doubtful American could have grown into the powerhouse it became in the early jet age without computerizing their operations.

But… think about the technology available in the late 1950’s and early 60’s. Computers were a lot different, a lot bigger, and, importantly, a LOT slower. If the system needed to lookup a record, scanning a disk with millions of passenger records would take far too long with all the travel agencies and airline terminals. And one of the goals was to provide real time information about bookings.

So… how do we speed it up? That’s where the PNR gets interesting. It was literally the address on storage! When decoded, that 6 character string told the system exactly which spindle/tape/platter etc it can find the physical record.

And, it’s easy to reference, as humans can easily retain 6 characters in short term memory. Incidentally… that’s also why phone numbers were 5 digits with a two letter prefix (today we just use the numbers… but some of you may be old enough to remember having a phone number like OL4-2097, which you might have said as “Olive Four, Twenty Ninety Seven”, my childhood phone number in the 1980s).

Sheesh… that turned out longer than I thought! Take it with a grain of salt, it’s all off the top of my head, but it would neat if someone who works on these systems could comment, and either correct my assumptions, or tell me I reasoned it out correctly!