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The correct choice, which identifies a type of train that consists primarily of freight and is not classified as a passenger train, is freight train. A freight train is specifically designed for the transportation of goods and cargo rather than passengers. These trains can carry various forms of freight, including bulk materials, containers, and automobiles.

Freight trains typically have much different operational characteristics compared to passenger trains, such as longer lengths, different braking systems, and often lower speeds. The design and function of freight trains prioritize efficient cargo movement over the comfort and scheduling considerations that are important for passenger services.

Other options like a deadhead train, which refers to a train operating without passengers to reposition itself, and M-Series equipment, which pertains to specific metro rail cars, do not inherently signify a freight operation. An engine alone, while crucial for any train's operation, does not fit the criteria of being a type of train in itself since it lacks the carriages necessary to transport freight or passengers. Therefore, freight train is the appropriate classification for a train primarily used for the transport of freight and not passengers.