The SL&RG is a basic short-line operation, collecting local freight and feeding it to a larger system-- in this case, the Union Pacific Railroad. SLRG's system is basically a huge Y, with the termini (terminuses? terminae? terminators? I'll have to look this up) at the towns of Antonito in the south, South Fork to the west, and Walsenburg to the east. It's somewhat more complicated than that, but you get the general idea.
Operationally, most traffic comes online or originates at Monte Vista and Antonito. Monte Vista traffic is agricultural, with barley and potatoes coming in from the even-smaller short line San Luis Central (SLC). Unlike the SLRG, the SLC actually rosters a large number of freight cars, mostly covered hoppers and mechanical refrigerator cars. SLC delivers cars to the small yard at Sugar Junction, just east of Monte Vista. The SLRG's Monte Vista turn picks up these cars and takes them to Alamosa's East Yard. There is also Poole Chemical and Wilbur-Ellis near Monte Vista, both of whom receive carloads of ag chemicals.
Antonito has two customers who account for the lion's share of carload shipments on the railroad. Harborlite Corporation's perlite plant ships-- you guessed it-- perlite, and there is also a scoria operation there that ships lava rock in open hoppers. See here for views of these operations prior to the creation of the SLRG proper.
There are other seasonal shipments, most notably incoming tank cars of magnesium chloride solution for CDOT's use in de-icing highways.
Basic operations look like this. Incoming cars arrive in Walsenburg on a UP local from Pueblo. The SLRG picks these up during the night and shuttles them over La Veta Pass to Alamosa. From Alamosa, two local trains are made up, one for Antonito and one for Monte Vista. The two locals go out, drop cars and pick up cars, bringing these back to East Yard. In the afternoon a train is assembled of outbound cars, and this heads east sometime around 5:00 PM (give our take a couple of hours). This train will have most of the railroad's locomotives, depending on its size, and will cross the pass in darkness. The outbound cars are dropped in Walsenburg, and the whole thing starts over again. This is mostly a Monday-through-Friday pattern, with some exceptions.
Of course, there are additional trains that run as required-- far more than when the line was under Union Pacific's control. And during the summer, the passenger operations add considerable variety.
For a look at the types of rolling stock used in the valley, see this page.
The SLRG interchanges with The San Luis Central RR at Monte Vista. This little railroad performs switching chores for a number of agricultural customers between Monte Vista and Center, and is actually a pretty busy line. The interchange traffic is handed off at Sugar Junction, which consists of a small yard running to the north, with a wye connection to the SLRG main.
In June 2006 the SLRG handled several trains of empty intermodal cars, taking them west to South Fork for storage. The Wagon Wheel Gap technically owns the tracks where they were being stored. I passed through on 6/30/2006 and photographed the cars. There were miles of these mostly spine cars (TTUX) in storage, stretched out along the line from the track to Creede through town, and nearly to the highway rest area to the east. They were broken at each grade crossing. As of the end of the year, the collection of cars had grown until it extended nearly to Monte Vista-- a distance of close to 30 miles! These cars were later scrapped at a location near Fort Garland.
However, the practice of using "spare" trackage in the valley for storage has continued. Over time one can see different kinds of cars parked on this line, as well as on various other spurs.