This is a list of car type codes issued by the General Superintendent of Transportation in conjunction with the implementation of the COMPASS computer system circa 1967-68. Both the GN and the NP purchased rights to the system from Southern Pacific, which developed it with IBM at a cost of several million dollars and called it TOPS, Total Operations Processing System. I believe that the Burlington and SP&S adopted it about the same time. By having all components of the proposed Burlington Northern adopt the same software before the merger, the training and learning curve issues associated with the new computer system were worked out before the merger. A very good thing.
The car type codes were included in train consists and switch lists immediately after the load/empty status. The first items on each line were thus: GN 10060 L B2. Code B2 was a box car of 40 foot inside length with a single 6 foot wide door, the typical grain and lumber box car of the day.
I use the GST code as the first part of the product description in the store. You can use it as a search term in the store.
The full list is published in the joint archives site under the GN Misc. heading. It is about 70% of the way through the list of items, second after a list of several GNRy List(s) of Equipment. Web address is http://www.gn-npjointarchive.org/GN_Misc/