Large Tank Cars

I've noticed that when the KCS passes near where I work, there are usually a few very large tank cars in the train.
They are about twice as long as a normal tanker and have two trucks at each
end. They are usually mixed in with other tank cars.
Does anyone have any idea what these large cars are normally used for?
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wrote:

see http://railwhales.railspot.com/a-roster.htm
Typical commodities are chemicals used in manufacture of plastics, by- products of chemical manufacturing, raw materials for fertilizers, etc. such as ammmonia, phosphorus, vinyl chloride, ethylene glycol (for antifreeze, polyester resin, etc), TDI, HMDA, etc.
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