The long trains you mention are called unit trains. They move from point A to point B as one unit. Typically the unit trains will be of all one type, e.g. tank, grain, container.
The trains with mixed types can be unit trains but they can also have intermediate stops where cars are added or dropped off.
The optimizations the rail companies perform to maximize the freight movement (and profit) is very interesting.
The trains with mixed types can be unit trains but they can also have intermediate stops where cars are added or dropped off.
The optimizations the rail companies perform to maximize the freight movement (and profit) is very interesting.