Re: About that connection...

"I recall that the D&SNG may still be a common carrier, but isn't CATS a historical tourist railroad only?"
Since the Staggers Act of 1979, the notion of "Common Carrier" doesn't exist in any practical way, as railroads are no longer regulated as a utility.
A "common carrier" is required to provide a product or service to any customer for the rate filed in its tariff, regardless of the cost of provision, in return for average rates that guarantee a profit. Every traditional "utility" - railroads, airlines, telecommunications, energy - all have been deregulated to some extent, so "common carrier" is a term that should be struck from the dictionary.
"Many of the requirements are not FRA, but AAR . . . The state(s) may have additional regulations."
1) The C&TS is subject to FRA jurisdiction for locomotives, cars, and track. Prior to 2000 FRA did not exert its authority on track, but did so for locomotives and cars. AAR is not a regulatory body. Over the years FRA has "red-tagged" historic freight cars for loose hand holds, inspected steam engines and passenger cars on a surprise basis, and track as well.
2) (For William) Connecting to the standard gauge at Antonito would change NOTHING. Track is under FRA jurisdiction if ANY of the provisions are met, such as: connection to the rail network (no at present), OR a public rail/road overpass/underpass (no at present), OR a public grade crossing (yes - at least nine).
3) FRA jurisdiction EXPLICITY exempts state authority. If your track and locos are under FRA inspection, your local Public Utilities Commission or other regulatory body has no standing. (D&S did submit RATES to the Colorado PUC until 2001, but locomotive inspections and trackwork were FRA - again, because of public grade crossings and underpass/overpass).
JAC