Difference in types of Ballast? - Trains Magazine
There are huge differences among types of ballast. Mud, sand, cinders, pit run gravel, limestone, slag, granite, and basalt are or have been reasonably common at one time or another.
Ballast is there to spread the weight of track and equipment to the subgrade. The materials listed do so in approximate order of increasing effectiveness. You want particles to interlock to privide more solid structure and to resist lateral forces. You want material to last forever in that locked condition. You want it cheap and located where you are.
Mud, sand, and pit run gravel particles do not interlock very well and are used today only on way back tracks, if at all.
Limestone was popular in the South because it was more available locally than the others, but it is a weak material that degrades from crushed rock to round pebbles fairly quickly. The result is that it degrades in terms of support and also in terms of drainage/permiablity.
A main line today will likely be slag, granite, or basalt. Slag was used when available. The slag that I am most familiar with was copper or copper/silver gold. The former on SP in Arizona and the latter on NP in Montana come to mind. I don't know if steel mill slag was popular around steel mills. I have heard a story that one line has been kept intact out of service out of fear that the slag will cause the right of way to be declaired a Superfund Site. I can neither confirm nor deny the story.
Granite is a light colored intrusive igneous rock. Exposures of granite are correlated with the cores of mountain ranges. One of the largest is the Sierra Nevada in California. Granite is most likely to be grey, but the Great Northern, now BNSF, has some very pretty pink granite basalt in Montana. Stone Mountain GA is also granite.
Basalt is dark colored extrusive igneous rock. The largest continental basalt eruption in the western hemisphere is the Columbia River Basalts of Eastern Washington, Eastern Oregon and Southern Idaho. Much of BNSF and UP in the Pacific Northwest is CRB. BNSF has a large basalt ballast pit at Mesa WA. In New England the chemically same material is know as "taprock", but the material may not have erupted in the surface. If it did not erupt it has a different geolgoical name which I can not recall at the moment. Taprock is the common name, not geological name. IIRC the Palisades in New Jersey are taprock. I know my PNW geology better than New England.
This is very broad brush treatment of a complex subject from both engineering and geological perspective.
Mac
