Somewhere in the multitude of comments about RTR non injection molding [
NIM ] frt cars was a suggestion that there might still be a significant frtThe ARA XM-1 seems to me to fit the bill. Multipule roads and with a little creativity it could be developed with multipule ends, doors, roofs and underframes that would correctly match many cars and be acceptable standins for several others. I knwo I would end up purchasing quite a few.
GATX 1920's tank car aka Sunshine and even the X-3 should be considered since they were found everywhere. You can even paint a 6500 gal X-3 up Durango and Silverton and have an accurate paint scheme for the toy market.
Also I agree with the Enterprise GS Gondola.