reporting marks with an "X" on the end are private owners, UTLX= Union Tank Car Leasing, LLPX is Something Something Locomotive Leasing, HELX is Helm Leasing, etc... although i have seen cars used for MOW with "X" at the beginning too
Helm leasing is HATX, HLMX, and HLCX.
LLPX is Locomotive Leasing Partners.
The letter X at the end traditionally meant a railcar owner, any railcar owner whether they are a railroad or not, but Class 1s cannot end with X. (CSX wanted the reporting marks CSX but because CSX is a Class 1, they had to use CSXT) Today, although most companies follow the tradition, the X at the end is not required.
Reporting marks can be 2, 3, or 4 letters, but not any more or less. Reporting marks cannot contain any punctuation although & is acceptable. If the reporting marks have an & in it, it is only on the car, and not on paper. Reporting marks cannot have any obscenities in them although "SEX" is allowed. (For example, Schneider Steel Company is SSEX)
When a railroad or company buys another company/railroad, they also inheirit the reporting marks. (for example, ARMN belongs to UP)
Examples of 2 letter reporting marks:
UP - Union Pacific
BN - Burlington Northern (now BNSF)
CN - Canadian National
CP - Canadian Pacific
ST - Springfield Terminal
BM - Boston and Maine
NS - Norfolk Southern
NW - Norfolk and Western (now Norfolk Southern)
CR - Conrail (now Norfolk Southern and CSX although NS owns the exclusive rights to the reporting mark)
WC - Wisconsin Central (now Canadian Pacific)
IC - Illinois Central (now Canadian National)
Examples of 3 letter reporting marks:
GTW - Grand Trunk Western (now canadian national)
SOU - Southern Railway (now Norfolk southern)
PRR - Pennsylvania Railroad (now Norfolk southern)
NYC - New York Central (now CSX)
CB&Q - Chicago, Burlington and Quincy (now BNSF)
CNW - Chicago (and) Northwestern (now Union Pacific)
MEC - Maine Central
MMA - Maine, Montreal, and Atlantic
NHN - New Hampshire Northcoast
Examples of 4 letter reporting marks:
CSXT - CSX
MBTX - Massachusetts Bay Transportation Authority (the engine I'm standing on in my avatar)
BNSF - Burlington Northern Santa Fe (Big Nasty Santa Fe)
ARMN - American Refrigerator Transit Company (now Union Pacific)
AMTK - Amtrak
LLPX - Locomotive Leasing Partners
LMSX - Locomotive Management Services (was a Conrail thing. No longer exists. I think NS owns the reporting mark now)
HLCX, HLMX - Helm Leasing/Helm Financial
HATX - Helm Atlantic Leasing
NYSW - New York, Susquehanna, and Western
MPEX - Motive Power Industries
EMDX - EMD leasing/EMD Test locos
GECX - General Electric Company