I have always wondered if there was actually a difference between different types of ballast. Such as the traditional gray ballast and the reddish type of ballast used by BNSF on the Cajon Pass. I am not sure what it’s technical term is.
The differences in ballast is determined by the source of the rock used. I assume railroads have a standard for rock size, and that rock is either screened or crushed to meet that standard. The CNW used pink ballast because it was an on-line source and crushed to size. The MILW in Montana used what looked like screened river rock, so it all depends on the railroad and existing sources.
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
Really good overview of this question. One small note, but you may have mistyped the name. It is Traprock.
Historically, the ballast a railroad used was what was available on line, hence limestone in the south, ganite in the west, cinders or dirt anywhere, etc, as you note. As demands for a better track structure developed, many things changed. Ties got bigger, rail got heavier and the need for a good solid base to build it on got more critical.
For technical requirements for modern ballast materials, AREMA is the governing source. Committee 1 is Roadway and Ballast and their chapter in the Manual for Railway Engineering has all the detail any techno-geek could want. There are several gradations to choose from but on most main lines you will see rock from about 2" down to about 1/2". It will have all fractured faces (run through a crusher but not from river rock which would leave smooth faces, for example) so it will interlock to make a strong, stable mass.
There are specific strength and performance requirements for the various types of rock and there are many ASTM tests used to measure these qualities. Some of the discussions in Committee 1 about what tests to use got interesting when you noted that some people were defending a particular test which showed their local material (be they from a railroad or a supplier) more favorably than another test.
Basically you want a material that will stand up to grinding action of trains passing without producing too many fine particles that could clog things up (drainage is the holy grail of good track), doesn’t fracture in freeze-thaw cycles, breaks in somewhat cubic shapes rather than thin and long, doesn’t have a lot of clay or dirt mixed in, etc.
This is where material like Traprock (New England and parts of the midwest), Quartzite (CNW’s pink lady at Baraboo, WI) and Granite (everywhere west) do better than Limestone (alth
The geological name for that ‘taprock’ is gabbro.
IIRC much of the former WP in eastern Nevada has slag from the Bingham Canyon copper mine as ballast.
Good overall summary by Mac, steve14, and corwinda.
Slag was indeed popular around steel mills. I worked with 2 basic kinds:
Open-hearth slag, which is similar to granite or basalt in weight, color, and durability - kind of like abstract steel pours, very dense and hard, a good though not an excellent material.
Blast furnace slag, a/k/a ‘popcorn’. It has a lot of air voids in it, so is much lighter in color and weight and breaks down much easier. Fine for industrial sidetracks, but not for mainline use.
Here in southeastern Pennsylvania, we have wonderful traprock and limestone quarries. ConRail used to haul traprock from the John T. Dyer quarry in Birdsboro (southeast of Reading) as far west as Indiana; Amtrak hauled traprock from the Glen Mills quarry (west of Phila., southeast of West Chester)as far northeast as Connecticut. With those sources available, limestone was used for industrial tracks, where it was well-suited to the needs and loads.
- Paul North.
What you’re looking at on Cajon:
“Newberry Ballast” = granite with quartzite flakes from Newberry Springs, CA (20 miles east of Barstow)…starts pink to red to grey as the oxide dust from the crusher washes out
Lets not forget the clam and oyster shells of the Old Colony RR.
Pete
So what your saying it will turn gray eventually?
Steel mill slag was used extensively where it was available, it was cheap and trouble free until the carriers started using electric signalling with current running through the rails to form signalling track circuits. The residual iron that remained in the slag, under damp conditions began conducting the electricity and creating problems with the signal system. Over the years most main tracks that once had slag as their ballast of choice, have had it changed over to other non-conductive forms of ballast.
What would be in the slag that would cause concern?
Steel mill slag is quite inert. In my neighborhood it was routinely used to surface alleys. The alley behind our garage had slag cinders for the surface and after several years of service, it was almost as hard as concrete and you could bounce a basketball off of it.
The problem implied in PNWRMNM’s post is not the slag but some other materials from the steelmaking process that may be in the ground.
Here in the boonies of Western Indiana where the NS runs, slag is found in older ballast all the time. (Late 1990‘s era ballast is still in use here). It is mixed with limestone and some basalt I believe. I think that as the need for new ballast comes around, NS is just using just a plain granite for the ballast, and covering up the slag mix.
Justin
Rereading this excerpt I see that I was not clear. The slag in question was not from steel; it was from a copper/silver/gold smelter. I would guess, and this is only a guess, arsenic or some heavy metal.
Mac
Has to happen sometime. Right, old timer?
Having that out of the way, I thought I’d mention the ballast conditions of the Pere Marquette Railway in 1944 (as gleaned from a very descriptive book on the PM, published by the C&O Historical Society–modesty prevents me from naming the editor):
Over one-third of the railroad’s main and branch-line track mileage (632.9 miles) was ballasted with pit gravel.
Almost as much (536 miles) had washed-gravel ballast.
Next was “stone” ballast, 276 miles. This was the wave of the future for the PM, with over 70 miles of track being re-ballasted with stone in the preceding year. In Michigan, this may be assumed to be limestone.
Cinders ballasted 142.6 miles of PM track; slag another 12.9 miles. Amazingly enough, 191.7 miles were ballasted with sand–or, putting it bluntly, no ballast at all (that figure went down by 37 miles in the preceding year).
As Paul North indicated balast used by railroads was generally the most available material (understand as probably the least costly product). Area by area, the material most convienient. In Yards, Shops during the steam age, it was cinders. Still to be found in the U.P.'s Parsons,Ks yard (nee KATY RR main shops facilities) was cinders and limestone mix. The KATY got most of its ballast from two resources, a quarry at South Mound Kansas ( on the former main from Parsons towards St. Louis, and another quarry at Stringtown, OK. The limestone at these two sites was extremely hard, and suitable for ballast use, ( the UP still resources ballast from the Stringtown quarry.) Just to note, a few doors down from my house, a former MoPac branch still has cinder ballast showing after more than twenty years.
Paul (CSSHEGEWISCHE) mentioned cinders in an alley. For years the most available crushed product in the SEKansas, NE Oklahoma, SW Missouri areas was the waste from the lead and zink mining operations in the area; it was free for the hauling off. &nb
Eventually it turns darker red and goes to gray-ish. Pedernel (NM) changes colortone as well (reddish brown to grey). The fines (" 3/4"chips" and “waste”) used in walkways and yard tracks turn color at a slower rate. Granite Mountain (UP & BNSF Laramie WY) comes out light gray to dark gray.
In Colorado, ATSF/MoPac/DRGW used Pueblo Slag for years. It fell out of favor because it crumbled to dust rapidly and then the steel mill (then CF&I, now Gervaz RMSM) changed from raw steel production using the Bessemer process to using arc furnaces and recycled steel. Used to be fum watching the molten slag ladlecars and bottles dump their loads on Minnequa hill.
Darn! I had to look some things up. I thought granite was a metamorphic rock, and I got a B+ in Geology 101/102 at a “Hard Rock” school, St. Lawrence U… I think ‘gabbro’ is too specific for all granite, though. Doesn’t gabbro have to contain some metalic ingredients? When I lived in New Rochelle, NY the only industry (siding-wise) in nearby Larchmont, NY was the New Haven Trap Rock Co… Quite a big operation, served by New Haven electric motors. I think the home-base was what is now the North Branford ‘Tilton’ Quarry, just east of New Haven, served by the Branford Steam Railroad, and still in operation. Out here, the Great Northern Railway had a quarry up near Essex, MT. It produced a cool pinkish/red ballast. The pit has been closed for some time, but the ballast remains in some areas, especially in the local’s planting beds (mine included!). Down in Sydney, NS it was slag-slag-slag, a “free” product for the CNR from the Sydney steel mill (Dominion Steel and Coal). As they say in the real estate business…
Hays
Gotta love that CNW pink ballast.