Sam,The Equal Service Clause is a non-issue in this instance. See item 17 of the lease agreement called "Service of Terminals". The freight would not be a problem.
I don't think this would hurt other freight charters since this one only covers less than 1/3 of the C&TS line(Cumbres-Osier-Cumbres). IF anything it might create an awareness that such things are possible, and plant the seed for future trips by groups or individuals.
Mr Carson suggested below adding cars to the passenger train and running a short freight back to Chama. In theory this looks good except that you may not have the capacity to add the extra cars going up the hill. The short train could act as a Roundtrip for Cumbres-Chama riders instead of a bus, but the idea of doing runbys with this type of freight on the downgrade would not have much appeal in my opinion.
Maybe instead of all this we just need a big steamfest like Durango in 2001 or 2002...Sounds good to me!
Greg Scholl