If you are referring to trains as a type of transportation (I can't think of anything else), then the separate "boxes" are called "railroad cars" or "railcars" in North American English and "railroad wagons/carriages" in the UK.
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deadratThere are many types of cars that make up a train. One of them is a box car. Is that what you're thinking of?I suggested that earlier, but all I got was the forum equivalent of a blank look.
CalifJimI suggested that earlier, but all I got was the forum equivalent of a blank lookYou made my day there, CalifJim.
XerxesYou made my day there, CalifJim.