A shortline railroad can be anything you want it to be. There are examples of everything being done that could possibly have been done. Your library should have a book on the subject. It's a good one, read it.
For example, just north of you and a little east was the Tonopah and Tidewater, hauling gold and anything else out of Death Valley and that region. This little railroad did it on the cheap, and only bought used equipment. They touched the Santa Fe, so I imagine they got some pretty good deals from them. When the gold was gone, it disappeared.
Farther north and to the west was the Virginia and Truckee. They hauled silver from the mines and wood timbers to the mines. Virginia City was really rich and attracted a lot of passenger service. This one was built 'at once', so equipment was acquired new from everywhere it was obtainable at that time. Really rich people owned it and kept it running for another 60 years after any other railroad would have gone insolvent; kinda like a really neat toy train, only big.
Movers and Shakers don't follow paths, they make them for others. Create your own shortline, and develop why it exists. It's a lot easier than following someone else's rules.
-rrick