Actually the first operator of the San Luis & Rio Grand was RailAmerica. They ran it for a few years then got somewhat disenchanted with it after they had a runaway wreck in the east side of La Veta Pass. Not too long after that RA put it up for sale. That is when Iowa Pacific took over.
The pic above is a bit interesting in that is shows 2 units going to Antonito. Since the beginning of time, they used only one unit to both Antonito and South Fork. When they did away with cabooses, they had to provide space for the conductor to do his paperwork, so they added a second unit.
Going back to Jerry's pics, the Conejos River and San Antonio River bridges are very old and are true monuments on their own.
The Conejos River Thru Truss Bridge was originally constructed in 1886 and was installed across the Conejos River in 1917. Originally it had been Bridge 205B on the standard gauge mainline - I don't know where MP 205 would have been.
The Deck Plate Girder bridges over the San Antonio River dates from 1880 and is original to the location.
Now, think of what has crossed these two bridges over the years. They have felt narrow gauge Class 45 ten wheelers (or even smaller power) all the way up to SD90MAC4300's. Pretty amazing.
One has to wonder if there are any other bridges that claim this.