WHAT ABOUT ALL THOSE FREIGHT TRAINS? - All Aboard: The Complete North American Train Travel Guide - page 204

A train of tanker cars. Photo courtesy of CSX Transportation
Tanker Cars
You can identify these cars very easily. They're basically long cyl-
inders with a dome on top through which they're filled. Tanker
cars are used for carrying liquids that range all the way from
milk to petroleum to some pretty dangerous materials—chemi-
cals, primarily. Many of these cars are pressurized to carry such
things as carbon dioxide, which we think of as a gas (providing
the bubbles in soft drinks, for example) but which turns into a
liquid when it's compressed.
Flatcars
As the name implies, these cars are really nothing more than roll-
ing platforms with no sides or top. The basic flatcar traditionally
has been used to carry things such as logs (to sawmills), lum-
ber (from sawmills), and other commodities that can be stacked
flat and wrapped or don't need to be covered from the weather.
Some flatcars have high reinforced panels at each end, designed
to keep loads from shifting either forward or backward with the
movement of the train. The military uses flatcars to move tanks,
trucks, and other heavy vehicles. You'll often see very large pieces
of machinery being carried in this manner.