Today in the US there are a lot white poles with colored tops stuck in the ground. These are Poly Domes. They are used to inform and warn the public about any delicate infrastructure that is buried beneath, such as gas pipelines or communication wires or fibre optics. Specifications and examples of the real thing can be found at ACP International including the specs of poly-dome-markers.
Lance Mindheim mentioned them in an article about vertical elements. Cody’s Workshop on MRVP shows a nice tutorial how to build right of way details such as poly domes.
This is indeed something to keep in mind when detailing the scenery around the tracks.