Richard Townsend wrote:
Based on that, I don't think the reporting mark is ?ZWX. I think it is SCMX. Back to front, I think we all agree that the last letter is "X." Rather than a "W" I think the next to last letter is an "M." It's easy to mix up a "W" with an "M" in a blurry photo given the similar strokes in the letters in this "font." The next letter to the left looks like a "Z" but it has that problem of reaching above the other letters and being quite degraded in the image. I don't think one can make a call on this letter based just on its image in this photo. Given that this appears to be a Shell Chemical Co. car, and that company has SCMX reporting marks, I'm going to say it must be a "C." And the first letter is obscured, but again given Shell Chemical Co., I will say it's an "S."
I would agree with all of this. In my 1953 ORER, there is only one reporting mark listed for Shell Chemical (SCMX), so it's hard to see a likelihood of any others. And I can't resist pointing out that there WAS a registered user of ZMX, the Zummo Meat Co. of Beaumont, Texas. But I don't think this tank car is a Zummo car.