This is not based on any formal research, but my understanding is that
"previews" are called "trailers" because back in the days of movie
palaces, lengthy film programs, and continuous performances, previews
were shown after the feature.  They "trailed" it.  Although this practice
ended in the sixties with the coming of suburban theaters and
noncontinuous showings, the term stuck.  However, my sense is that
"trailer" is an "inside" industry term; popularly, they're previews.
 
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