From
http://www.pwrr.org/rrm/index.htm:
MOOX MIDWEST TERMINALS OF TOLEDO INTERNATIONAL
I have the same problem with my narrow gauge. The mining company also owns the tank cars, and provides fuel shipping services along the line. I chose MORX for the initials years ago, and that turns out to be two different companies, although I don't know if either were in use in 1974.
MORX *JOHN MORRELL & CO. (NACCO [MATHER STOCK CAR CO.]) (no longer used)
MORX +MORRIS IRON & STEEL CO. INC. (current user)
My mining company is MMCX, which is listed as
MMCX *MCCLINTLE-MARSHALL CONSTRUCTION CO.
MMCX *NORTH AMERICAN CAR CORP.
MMCX +PAN AMERICAN RAILWAY CO. [(TMIX) TRANSMATRIX INC.; M & M CHEMICAL PRODUCTS INC.]
I got luckier with the railroad itself, as neither WRRY, the railroad, or WRLC, the logging company that own it, are listed as ever being assigned.