It says "10 car Texaco tank car fleet on siding at Antonito" 1950 by Bob Richardson. Note the single domers too.
I've got to go home and look up what Sloane says, but due to the number of photos that I've seen (like in the Narrow Gauge Pictorial), I'd think that you would have had a far greater chance of seeing these guys on the western slope and Gramps cars almost exclusively on the eastern slope. Maybe not. Maybe the Gramps and plain UTLX cars so outnumbered the TCX and CONX, that it creates that impression. I don't have any records of freight movements.