Hi all----The size of the car fleets generated in mergers make it easier to use "merger" reporting marks instead of having to shuffle cars around to create full blocks of cars in the reporting mark of the absorbing line. That is a central reason CR used the PC car numbering scheme as the core of its car-numbering plan, the fewer cars that have to be renumbered and relettered the better, so using the largest fleet as the core set of numbers simply makes sense. When the core groups is very large and the number series do not allow for simple "insertion" of newly remarked cars, you run into problems like CSX has. The large car fleet picked up from Conrail simply could not be "folded" into the CSX numbering system without a cascade effect of other pre-existing CSX car groups then in turn having to be renumbered to make room for the new X-CR cars.It is more cost effective for CSX to use the NYC reporting marks on former Conrail cars that to have to entirely re-do their rolling stock fleet numbering system to accommodate them. CSX, in fact, used a common numbering system for locomotive purchases from 1983-1986 (when Seaboard System and Chessie had not yet been merged together) in anticipation of the coming CSX combination. C&O and B&O did the same under the Chessie regime for locos, but continued to letter the cars for B&O, C&O, and WM. Loco fleets are tough enough to make common, but car fleets approaching 100,000 are VERY difficult to remark into a common plan without massive errors or overlaps among series, things that will make computers choke.
UP, for example, not only has continued to use WP, MP, SP, CNW reporting marks, but has used subsidiary reporting marks for specific purposes. My understanding is that the CHTT (Chicago Heights Terminal and Transfer) reporting mark, formerly a subsidiary of the MP, is now used by the UP to designate leased cars.
NS did the same thing, leaving most X-Southern and X-NW cars in original reporting marks. As they purchased or leased new cars, they went into a common NS reporting mark scheme. As the older Southern and NW cars were either sold off or retired, the number of cars requiring renumbering slowly became smaller. Further, rebuilds of hoppers allowed them to be moved into new series indicating their new capacity and service. As those things went on, the NS renumbering scheme became a smaller, simpler task.
Decisions like this are driven by cost and the effectiveness of the simplest solution.
I also wrote an article about roster construction for proto/freelanced lines. It can be found here: