Of course, while crushed stone (often limestone or quartz) is the aggregate
of choice for today's railroads in years past everything from slag to
cinders has been used (always resourceful years ago railroads would use
whatever they could find). Some light density railroad lines would
appear jet black as nothing but burnt coal cinders were used to ballast the route. In any event, ballast must regularly be cleaned or added as when dirt and grime builds up within the rock it reduces its ability to properly drain water. Ballast also acts as a support base for the railroad track structure
giving it strength and rigidity but also allowing for flexibility when
trains pass over. Limestone or quartz is often most used as ballasting because it is a hard stone that will lock together providing for extra strength. According to Brian Solomon's book Railway Maintenance, The Men and Machines That Keep the Railroads Running,
today's trains can exert a force of 100 psi when passing over track and
the stone used for ballasting must be able to withstand this constant
For history's sake ballast has its earliest beginnings as simple limestone blocks, which actually sometimes pulled double duty as both the support base and railroad track structure. In the 1840s true ballast, or crushed stone, as we know it today began to be widely used and was soon found to be far superior to the old method. Years ago during when most of the classic railroads were operating each had a particular quarry in which it used for ballasting.
Of course, back then most railroads were small enough that a single
quarry could often fulfilled the needs for the entire railroad. Because
of this many railroads could be distinguished by the stone it used to ballast its system.
One often forgotten aspect of the ballasting system is what is known as sub-ballast
or sub-grade. This layer of crushed stone or even pavement, as some
railroads today now use acts as a moister barrier and added support
system for the railroad track structure above (including the rails, ties, and ballast).
It is always the first component of the track structure to be laid
down and is a very important, unseen component. If the railroad track
did not have a sub-ballasting system, or very poor one at best, the rails and railroad ties would eventually become water-logged and fail (causing a derailment, or worse, a washout).
According to Brian Solomon's book Railway Maintenance, The Men and Machines That Keep the Railroads Running, today the typical layout for a well-ballasted railroad track system is stone lined to the top of the railroad ties protruding roughly 14 inches to either side, tapering away on a "3-to-1" slope to an eventual distance of 9 feet, 10 inches from the center-of-track with the stone being about one-foot deep below the bottom of the ties. Pretty technical stuff for something that looks so simple!
If you ever happen to take a closer look at a well-maintained railroad right-of-way be sure and notice how the ballast system works, aside from the stone and technical layout. To either side of the railroad track below the ballast you will notice a well-groomed ditch that carries away all of the water which drains from the railroad track structure. Also pay attention the rails and railroad ties. If the system is properly ballasted and maintained no water will be anywhere near either, as will weeds and other foliage. It's hard to believe that simple crushed stone plays so many vital roles as part of the railroad track structure.
For more reading about ballast you may want to consider the book Railway Maintenance Equipment: The Men and Machines That Keep the Railroads Running
from noted author Brian Solomon. While Solomon's book primarily
focuses on railroad maintenance equipment across its 128 pages it has a
chapter devoted entirely to ballasting equipment used by railroads and
specifically discusses ballast itself. I own this book and have
used it as reference material for this site many times. It's a great read highlighting an often little understood and/or overlooked area of the railroad industry. Without these vital machines the trains could not run!