Ted
Am a little surprised that Clark did not add this to his answer. Lots of
ag lime is mined right here in Iowa, as in any state with limestone
deposits. It is spread on soils that are to acidic. Early limestone I
believe was hauled in gons and hoppers to crushers, then (like cement)
in boxcars, bagged or bulk. Today most is handled in covered hoppers, or
more likely covered trucks. A current check shows over 150 active
Limestone companies in Iowa alone. They produce limestone gravel,
powered ag lime, lime for cement production, etc.

Here is a link to the history of limestone in Iowa:
http://freepages.history.rootsweb.com/~quarries/states/iowa.html

As to Ag Lime the following is from:
http://www.soils.wisc.edu/~barak/soilscience326/may07_02.htm which also
has some interesting photos of early lime application.
"Soil Acidification and Liming: Causes and cures
Many soils are naturally acidic as a result of the pedogenic processes
that formed them. All agricultural soils receiving input of ammoniacal
fertilizers, including ammonia, urea, ammonium nitrate, ammonium
sulfate, etc., are subject to cumulative acidification ....

Acid soils are weak acids, but unlike dissolved weak acids such as
acetic acids, the reserve acidity is in the solid phase instead of the
liquid phase. ...

The remedy for acid soils is adding base. Usually the base is a weak
base such as calcium carbonate, ....

Liming can be considered as part of sustainable agriculture for
noncalcareous soils but the mechanics of incorporation are more
difficult with conservation tillage practices. With moldboard plows,
aglime would be applied dry to the soil surface and then mixed
thoroughly, and relatively uniformly, with the plow. With minimum
tillage or no tillage practices, it is difficult or impossible to mix
aglime into the entire soil volume that needs pH correction and simple
surface application of aglime will not react with the underlying soil
with requisite speed.

Liming Materials
The standard of aglime is pure calcium carbonate, ... Aglime materials
are often sieved to various mesh sizes to analyze the particle size,
with each sieve size assigned a relative value reflecting its
reactivity, ...

In addition to calcitic limestone, "dolomite" [=dolostone] or dolomitic
limestone containing the mineral dolomite [CaMg(CO3)2 or CaCO3MgCO3] is
often used as a liming material, ...

Other liming materials are waste products from other industries. For
example, spent lime from chimney scrubbing operations designed to remove
S from emissions of power plants and incinerators still contain
considerable neutralizing power. Fly ash, slag, kiln dust, and papermill
slurry are other liming materials that wastes that would otherwise have
to be disposed of in landfills. ...."

Now freight car fans, as I said, I assumed how the traffic was handled.
But truly, what kind of cars transported ag lime? Any Photo's?

Doug Harding
Iowa Central Railroad
www.iowatelecom.net/~dharding/