Cabooses aren't usually interchanged anyway, and I've heard of cabooses over 70 years old being used. I can't say how long these cabooses in question lasted.
SUNX is a private owner, not a railroad. Thus, the tank car would never be running on its own tracks. Is it possible for a private owner to operate cars over 40 or 50 years old?
No, the rules apply to anyone operating in interchange service where the cars go from one railroad to another. Private owner cars and lease cars are usd entirely in interchange service.
There are 3 possibilities that I can think of.
#1. If you are not attached to the SUNX paint scheme, repaint the tank car in the CNW's mow scheme. Virtually all of the railroads made up their mow fleet from older cars no longer legal for interchange service.
#2. It is your railroad, do what you want the only rule is to have fun. The only problem with this solution is if you realize that what you are doing isn't prototypical and become your own "Prototype Police". You don't need to worry about what anybody else says. It is your railroad, but if it starts bothering you because it doesn't look right to you, you would need to change it.
#3. This one is my favorite, but it requires storage space and can be more expensive since you will want to buy everything instead of only those products that fit your railroad and era. Make only your scenery permanent. All buildings, and details are removeable. Now, you can model whatever era you wish by just putting on the period correct rolling stock, buildings, automobiles, and details. You could model the 1930's &1940's, 1950's, 1960's, 1970's etc. just by changing details. I lump the 1940's with other eras because with WW2 most equipment, automobiles and details up to 1947 would be 1930's era. Most of the 1947 model cars were new production from 1942 dies. The new body styles didn't come out until 1948-1949. Even though it is my favorite, it is not the method I use because I don't have the storage space for all of the stuff that fits other eras that I'm not running right now and I don't have the money to buy everything that comes on the market. My hobby dollars go a lot farther when I can look at a really neat new locomotive and say, "No thanks; it doesn't fit my modeling era."
I just thought of a 4th possibility. The CNW tended to run equipment for a long time, and the Alco rs32 wasn't bought until 1979 so under your scheme of things with CNW never sold and continuing on as an independent railroad, the rs32 could still be in use today. The f3 would probably not continue in use because the railroads discovered that road switchers were much more practical for everyday use than the "covered wagons", but it could be restored to use for a business train for the CNW. Since AMTRAK, many of the railroads have restored F &/or E units to go with restored passenger equipment to use as a business train. For instance BNSF & UP both will run their business trains with a guest list by invitation only to any super bowl city that is on their operating territory on the weekend of the super bowl. BNSF runs an annual employee appreciation special with the business train giving rides all over their territory for employees and their families. If you decide that cabooses were never eliminated on your version of the CNW, then the cabooss you have could continue in service indefinately. That just leaves the tank cars to deal with.