Train is used as an analogy to a railway train, made up of a "set" of carriages, each with a specific function (1st class, 2nd class, restaurant car, baggage truck, etc.). Except that this "train" has a succession of fans, filters, etc.

And each train (or set) is independent, in the hope that there'll always be enough of them working to keep the whole system operating safely.

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Note added at 59 mins (2016-12-06 20:12:32 GMT)
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Response to Asker's question below:

No. A "train" is a linear system, in no way similar to a loop. Especially in this case described in the ST - the exhaust gases will go through the "train" of fans and filters once, and then exit the system (the text says "... to the outside"). They won't go round any kind of "loop".