While Union Pacific's records for its cabooses and other rolling equipment show retirement dates up to about 1975 (with some sales), the records after that time are spotty and mostly incomplete, especially for sales. It was at about this time that Union Pacific began using a central computer for all record keeping. Most of the caboose donations were recorded, at least up to about 1995. After that time, records are mostly nonexistent.
The fact that UP's own equipment records are so incomplete is sad but true. In these days of no government regulation, and focus on increasing shareholder value, costs at headquarters have been severely cut, and the most expensive cost is labor, especially having clerks record and maintain information that the government no longer requires (car ownership and usage). Cars and locomotives are merely assets to be bought and disposed of, kind of like the little car that the guy at the pizza place drives; when he needs a new one, he simply goes and gets the cheapest one he can get, and does not need to answer to anybody.
The Federal Railroad Administration still requires the air brake information to be recorded on the blue card in the locomotives, but only for the most minimal of safety reasons. The original blue card is in the locomotive cab, and a copy of the blue card is kept in Omaha, but there really is no hard emphasis to get the previous ownership right, or to get the date information correct. There is no penalty from the FRA if the information is found to be wrong, except for a minor slap-on-the-wrist of a one-time $250 fine. It's easy on the new stuff, but the leased stuff and stuff from merger partners generally falls by the wayside. And there is none of this safety requirement on cabooses after the 1989 decision by the Supreme Court that found that states could no longer require cabooses on all trains.
Getting information about railroad equipment is very hard these days. It is amazing how many records have simply been destroyed. They take up space (that needs to be paid for), and there is only the most minimal government requirement for them in the first place.
Almost everything we know about Union Pacific from 1914 to about 1995 comes from the railroad's records that it kept to fulfill the ICC requirement for valuation of its assets. The ICC went away in January 1996, and with it went the history of railroading after that date. UP has disposed of almost everything prior to the magic seven years of required records retention, and they are very focused on that constantly moving date. Now, seven years later, anything before 1996 is gone. I have seen many, many boxes with this notation: "Throw away after [some date]" with the exact date being seven years after the record was initially boxed up and kept.
To return to the subject of cabooses; I have copies of records up to 1995 (retirements and donations, but sadly, no sales). After 1995, there are simply no records available, other than whatever some individual clerk may have kept at their own desk, or in their own files, to make their own everyday job easier. As these people have retired (and many have, with several buyouts having been offered), either the person themselves threw the stuff away as they left, or the railroad disposed of it to clean out the person's desk and files. I have gotten to know several clerks over the years, and they all have horror stories about missing records since 1995.
What does this all mean? It means that current and future historians are in for a rough haul.

