While it may operate in interstate service, is it still a common carrier? Was abandonment actually granted by the ICC after 1968? I recall that the D&SNG may still be a common carrier, but isn't CATS a historical tourist railroad only?
Be careful when you start comparing CATS (or any similiar) operations with a modern railroad as far as regulations.
1) Many of the requirements are not FRA, but AAR to allow interchange of cars (origins from the Master Car Builders/MC
. The FRA tends to "rubber stamp" many of these safety requirements. (There may be differences - AAR limits interchange to cars less than 40 years old; the FRA limit is 50 years for service, but allows railroads to create their own exemptions.)
2) The federal regulations that apply to railroads and are enforced by the FRA (CFR49), have specific exemptions for both historical equipment and narrow gauge. (Imagine a K-36 with ditch lights?)
3) The state(s) may have additional regulations.