I find that individual railroads use several different reporting marks before the number of the car. Example CN uses the following: CN, CNA, CNAZ, CNEU, CNEZ, CNIS, CNPU, CNPZ, CNG, CNRU, CNEZ.
Do these marks indicate what the commodity to be hauled in the cars, or what area of the country they are to traverse, or if cars derived from a takeover by CN rail?
CNAZ = Grand Trunk Western
All the others show-up as CN in my listing.
I know UP is reusing CMO, CNW, MP, and SP since they own those “fallen flag” reporting marks.
Don’t have time now, but will get into this later. Anyway, reporting marks ending in “Z” are for intermodal trailers (and container chassis); those ending in “U” are intermodal containers.
AC, ACIS, AHW, BCNE, BCIT, BCOL, BLE, CC, CIW, CV, CVC, DMIR, DTI, DTS, DWC, DWP, GBW, GTA, GTW, GMO, IC, ICG, NAR, PGE, PSL, SSAM, WC, WCCL, WLO are Canadian National reporting marks gained through more recent acquistions or were previously part of the CN family. Plus at least a dozen or more intermodal and many obsolete marks. 
I couldn’t tell you how CN decides to give to which trailers (TTTZ) or containers (CCCU). But this should take care of the rest of them:
CN: Darn near everything assigned to the CN proper.
CNA: Canadian National cars built in the United States (not all-inclusive–some US-built cars have been found to be lettered CN on occasion).
CNIS: Canadian-built cars used for international service only (this mark may have become obsolete with NAFTA or something, but CN never hardly ever reletters anything.
CNG: cars used on the Newfoundland narrow-gauge operation.
Now, all those reporting marks from Amtrak Jackson are assigned to components of CN North America (there are a number of those you won’t see on equipment any more). I think the Official Railway Equipment Register (I need a new one!) shows GTW, WC, and IC (at least) as separate companies.
Thanks for the information.
There is also similar reporting marks for the CP which I am also checking. When CP operated through the state of Maine for traffic from Montreal to Saint John, NB. There was a specific reporting mark for these cars, although they could be found across the system.
If you look up Burlington Northern there are about ten markings starting with BN. One of these is BNML which is designated for Burlington Northern (Manitoba) which you could probably find these cars across the system & not just confined to US/Manitoba consignments.
Not in my lifetime. CP has reporting marks CPAA for American-built equipment (corresponding to CNA on CN), and used to have cars lettered CPI for international service (definitely not restricted to the International of Maine Division), but most, if not all, of those cars have been relettered CP.
Again, this probably includes a bunch of trailers and intermodal containers. The most notable would be BNFE (used on its refrigerator car fleet). Of course, if you’re looking in a post-1996 era, you’ll find BNSF, too. There is no separate reporting mark for refrigerator equipment on the current railroad–everything’s lettered BNSF.
As for BNML, there are no freight cars carrying this reporting mark, to the best of my knowledge. I believe its use was restricted to one locomotive–which pretty much did stay on the Winnipeg portion of the line.