Regarding your first example sentence:
The journey to Vancouver takes three days by train.
This is the so-called generic use of the definite article, like in
The Siberian husky makes a good sled dog. (This statement is not about some particular husky. The is used generically to refer to the members of a class in toto.)
With a count noun like "Siberian husky" you have three options to express generic reference
The Siberian husky makes a good sled dog. (the + singular form)
A Siberian husky makes a good sled dog. (a + singular form)
Siberian huskies make good sled dogs. (no article + plural form)
However, the three options are in free variation only when "Siberian husky" is the subject in the clause. When we use such a phrase as object, it might not be generic in all three variations. For instance, only option 1 below will refer to "Siberian huskies" as a species:
- Geneticists have been working to improve the Siberian husky. (the species as a whole)
- Geneticists have been working to improve a Siberian husky. (some particular dog, not the species)
- Geneticists have been working to improve Siberian huskies. (most likely some subset of the species, for instance, a group of 100 dogs and their progeny)
Regarding your second example sentence:
∅ Furniture of that quality is too good for a student flat. (with "∅" denoting the absence of an article)
Furniture is a concrete noncount noun. "Concrete" in the sense that it could be felt by hand, "noncount" in the sense that you can't have 1 furniture, 2 furnitures, 3 furnitures etc.
According to Quirk et al., we can omit the before a concrete noncount noun, although we tend to add the when such a noun is postmodified by an of-phrase
This museum specializes in 18th century furniture (pre-modified by "18th century)
This museum specializes in the furniture of the 18th century (post-modified by an "of-phrase")
Quirk et al. write that the use of the in such cases elicits a slight contract: without the, we understand "18th century furniture" in the widest possible sense. With the, the sentence allows the interpretation that the museum specializes in only some kinds of 18th century furniture.
So Quirk et al. write that in these cases (with concrete non-count nouns postmodified by "of-phrases") it is to a lesser or greater degree acceptable to omit the, probably in order to impart the widest possible sense to the phrase:
The museum specializes in furniture of the 18th century.
Reference:
Quirk et al., "A Comprehensive Grammar of the English Language", Chapter 5.52, The articles in generic reference, and Chapter 5.58, The articles with abstract noncount nouns.
John Lawler, "Re: A question about the generic use of articles", in "Ask a Linguist", University of Michigan, May 1997
Best Answer
The rule says to use the plural to mean either "all" or "any" of that thing. You have to figure out which of these from the context.
Basic logic applies here. If a man has three apples and you say "The man is eating apples" it would be silly to assume he's eating all the apples in the world. The more reasonable assumption is that he will eat at most those three apples.
Of course he could eat one, or two, or all three of the apples. The exact number is unspecified, and unimportant. The point is that he is eating apples, and not something else.