The sentence

Since the train was delayed for more than an hour, passengers were given a full refund.

is formally ambiguous. One cannot tell from the sentence alone weather the passengers were given a refund -- that is, all the passengers. Or whether only some passengers were given a refund -- perhaps only those who complained. When an article is elided in this way, the reader must determine from context and common sense what the meaning is, and which article is implied. Here either choice is possible, although "the" seems more likely. But in another context the result would be different.

After the minister's eulogy, friends and family spoke about the deceased.

Does that mean that every one of the dead person's friends and family spoke, or only some of them? Were all of them even present? "Some" is the likely choice here, but furhter context could change that.

After the minister's eulogy, friends and family spoke about the deceased -- all five who were still alive.

Now the implication is otherwise.

When the choice of article is obvious, omitting it does not mislead the reader. When there is more than one serious possibility, this may be poor writing. Or it may be intentionally ambiguous writing.

The sentence

Since the train was delayed for more than an hour, passengers were given a full refund.

is formally ambiguous. One cannot tell from the sentence alone whether the passengers were given a refund -- that is, all the passengers -- or whether only some passengers were given a refund: perhaps only those who complained. When an article is elided in this way, the reader must determine from context and common sense what the meaning is, and which article is implied. Here either choice is possible, although "the" seems more likely. But in another context the result would be different.

After the minister's eulogy, friends and family spoke about the deceased.

Does that mean that every one of the dead person's friends and family spoke, or only some of them? Were all of them even present? "Some" is the likely choice here, but further context could change that.

After the minister's eulogy, friends and family spoke about the deceased -- all five who were still alive.

Now the implication is otherwise.

When the choice of article is obvious, omitting it does not mislead the reader. When there is more than one serious possibility, this may be poor writing. Or it may be intentionally ambiguous writing.