Looks as though the BNSF Railroad has an FAA waiver for using drones to inspect track and monitor train operations, as well as for other yet-to-be-named purposes. It took them three years to get it--
According to the article, the National Agricultural Aviation Association (crop dusters) requested some restrictions on BNSF's drone operations with the idea of preventing drone-plane collisions. The article doesn't say what the NAAA asked for, but these drones will be limited to 400 feet altitude and a speed of 50 knots. They must also be within the line of sight of observers on the ground.
It isn't clear whether these drones would be allowed to operate over BNSF tracks if they are near an airport. Probably not, but the article isn't that detailed.