I wonder if the reporting marks "NIPX" might possibly not be for Northern Indiana Public Utilities. In post-list years, the organization used "NORX", not "NIPX". And in my list-years of ORER, I can't find reference to either Northern Indiana Public Utilities or NIPX. And, although all cars with reporting marks are supposed to be listed, NIPX 8 is not; so an absolute connection between the cited organization and NIPX does not exist in the ORER's that I have.
So, I think I will speculate that "NIPX" is for a different organization--one that would have a likely use for a two dome car like OMI 3230. Since "P" also stands for plastics, I wonder if the car was used for transporting a related chemical. The car was reportedly built in 1930. If so; and if it were purpose built, the list of chemicals would be much shorter than it would be today.
I make the above speculation because I don't see a utility buying a tank car to have transformer oil delivered. Car utilization would be awfully low, I think. Why not have the manufacturer deliver the oil in THEIR tank car--one that would have much higher utilization since the manufacturer would ship an awful lot more oil. And, going the other direction, I'm not coming up with much in the load-out category.