You should know the lay of the land before you get there. Google maps will tell you where roads and tracks are, but you need to be on the ground before you actually railfan. Plan photo shoots before you start shooting.
Be aware of your surroundings. Watch for gopher holes so you don't twist an ankle, watch for snakes, watch for poisonous insects, don't bother little critters. If you come across a deer, take a photo or two, then backaway slowly. They'll run when they hear the train, but don't force them to. In panning shots, watch for trees that suddenly grow about 20 feet in the 2 hours you're sitting there.
Know your camera. Don't be caught fumbling with obscure settings when the perfect shot comes along.
Bring food and water (lots), you'll be sitting there a good while (unless you really know the schedule). Shade is nice too, and make sure everything is waterproofed. Mother Nature plays dirty.