The conventional clerestory roof is explicitly found in connection with the advent of the early (narrow) vestibules -- the clerestory part of the roof extending 'through' from end to end including the thinner door areas, curving down only at the extreme ends. Prior to that you see the "clerestory" being only a raised monitor-like section in the middle of the car, a construction that persisted into a few types of New York City transit cars.
As long as the passage between adjacent vestibules was restricted to the 'manway' dimensions between them, it made sense to put the curved end on the clerestory roof rather than provide 'through' streamlining. It might be interesting to look back at the original streamlined-locomotive patent (we covered it in a thread here which is now unsearchable) and at other 'windsplitting' patents to see which of them used arch roofs (which are better in 'quartering' streamlining) and which preserved clerestory lights and ventilators.
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