In 1999 UP adopted the UPY reporting mark for their yard switcher locos. The UPY reporting mark was necessary as UP was running out of 4-digit numbers with the incorporation of the C&NW/SP fleets and the 1000+ order of SD70Ms. I think if UP wanted too they could have simply did what CN done and simply kept the C&NW/SP/SSW reporting marks and road numbers for locos inherited from those railroads. Like CN, locos repainted into UP’s armor yellow could have the C&NW, SP or SSW reporting mark in sub lettering. By the way, UP adopted this method for rolling stock. Some UP covered hoppers even have reporting marks from railroads that were fallen flags since the 1960s.
UP only has 8,000 locomotives (http://www.uprr.com/aboutup/reference/locorost.shtml). They probably chose to add UPY rather than do more renumber. However, UP can fit all of its locomotives in one reporting mark.
While UP would have been able to get all 8000 engines under the same reporting mark, they probably would not have been able to get all the like type locomotive into the same series of number. Railroads like to put all of a single type of locomotive into the same number series. Doing so makes it easy for those who have to deal with power to know the kind of engine they are dealing with. ie. All CW44AC’s grouped within a series of number, All the Dash-8’s grouped within a series of numbers, all the SD-40’s grouped within a series of numbers. Railroad have used this system of numbering for eons.
A few things that I’ve noticed about the UPY reporting mark give it a bit of an advantage over the CN system of old numbers and reporting marks. The CN system can lead to confusion since the reporting mark is not prominent and the number can be duplicated with another reporting mark, CN and IC both have locomotives in the 6000 series. The UPY reporting mark is on the cab over a smaller-than-standard number (which is an eyecatcher) and the numberboards show a Y with the number: UPY 714 (a GP15-1) shows Y714 in the numberboard. There are multiple subsidiary reporting marks on CN (IC, B&LE, DMIR, GTW, EJ&E, etc) while UP has only the UP and UPY reporting marks on locomotives.
There are also supposed to be subtle differences in the paint scheme, notably some red-and-white striping on the noses of the UPY locomotives.
As it is, the units may be in large groups and fairly close together, but the ES44s are now numbered on both sides of the pre-GEVO AC4400CWs (never mind the “C45ACCTE” designation that covers all but the earliest of these), and the numbers for the Dash 9s have changed a time or two as well since the original post-merger renumbering plan. In a year or two the GEVOs will run out of numbers, and we’ll either see another renumbering or a fresh GEVO block somewhere. The SD70Ms were originally going to occupy the 4000 series, but have since overflowed that by a couple hundred or more units in both directions.
We could probably use a new locomotive model from somebody, to occupy the 2000 and lower 3000 series, when the 40s and 60s currently in these blocks are put to pasture.