The WRWK reporting mark is on an enormous number of railcars of all types around the continent - boxcars, intermodals, and I'm pretty sure I've seen it on lumber bulkheads as well. (I just drove around the U.S. for three months so I generalize with confidence about the continent.) These types of markings are clearly leased marks rather than the short line building their fleet, although I don't understand what advantage leasing a reporting mark confers these days. (I know it made a financial difference in the countdown to deregulation, e.g. Southern Pacific's gamble with the Golden West Service fleet and leased VCY, GVSR, and CRLE marks.) Another current example is COER (Crab Orchard and Egyptian), a tiny shortline in Indiana, whose reporting mark is on thousands of cars all over the place.
I'll stop here before I veer any further off topic.