This third-rail thing just won't die. First it was the Royal Gorge, now Antonito. The reasons for not putting a third rail into Antonito is the same reason the Rio Grande got rid of the third-rail, except for the Alamosa-Antonito section in the first place during the early part of the 20th Century.
Laying a third rail to Alamosa would mean a large capital outlay with a questionable return on investment. Switches would have to be replaced and tampers and ballast cleaners would have to desgined for three-rail track. I could go on as I have in the past.
Actually a third rail in Antonito may not be a bad idea so that Cumbres & Toltec Scenic Railroad trains and Rio Grande Scenic trains could use the same depot. Even then that may be accomplished without laying a third rail.