Tim Gilbert wrote

Another guess based upon the shape of the emblem would be the Diamond
Alkali Company but most of these 237 cars were intended for chlorine,
vinyl chloride or methyl chloride - ICC types 105 & 106. I believe that
tank cars carrying chlorine did not have expansion domes so that should
shoot down Diamond as the owner as a possibility.

http://home.comcast.net/~sseders/tankcar.html

Tim, that is a good guess, but the Diamond Alkali emblem was multicolor
and had writing through it that extended beyond the edges of the diamond.
I have a 1956 photo of a Diamond Alkali 103W with a large dome, with
reporting marks GATX 32991. In fact, I'm pretty sure it's the only car
I've ever seen that matches the Overland brass model #3229. Unfortunately
I don't know of any decals for it.

I would very much like to see a better photo of that 10k Dow tank car.
Dow leased cars so not all of them had DOWX reporting marks -- I have
photos of GATX Dow Chemical tank cars, for example. Dow was a big outfit
based in Michigan, not Texas, but they had chemical plants in several
states.

From Tim O'Connor

Looks like a Dow Chemical tank car. The Ohio, Kanahwa and
other river valleys in that region had (still have) lots of
chemical plants. Judging by the type of car (insulated 103)
I would guess it's for sodium carbonate or sodium hydroxide,
two common industrial chemicals.

Response from Tim Gilbert

In the April 1955 ORER, the only Dow cars listed were from their Texas
Division (128 cars - all ICC-103's having five or six digit numbers) and
3 cars from their Canadian Division based in Vancouver BC.

Response from Richard H. Hendrickson

Tim is correct that this is a Dow Chemical Co. tank car, in this case a 10K
gal. ICC-104 (or insulated ICC-103, which amounts to the same thing) built
in the 1920s by the Standard Tank Car Co. - note the distinctive sloping
box bolsters.