A couple of readers were puzzled by that "may have been." "I hadn't realized that the Silver Line was missing," e-mailed David Devore of Newton. William F. Bell of Lenox, Mass., agreed: "The meaning is unclear unless you realize that what is meant is 'might have been placed."
I'm with them. For me, the verb there can only be might, the past of may. ("May have been" means there's a chance the line was once placed there; we know that isn't true.)
But the distinction seems to be evaporating. Just two days earlier, I had spotted the construction on the New York Times op-ed page: "If dentists would just decide to withdraw the flossing directive, we may have enough additional spare time to learn Spanish." I could go with either "If dentists decide, we may," or "If dentists decided/would decide, we might," but as written it sounds wrong. This may/might choice is not about levels of likelihood, just about sequence of tenses; normal English uses "She said she was happy," not "she said she is happy" (unless, some say, you intend to emphasize the latter verb).
And today the Globe's op-ed page has, "I fought off the temptation to shoo the animal with a firm 'no!' or 'go to your bed!'’ -- commands that may have gotten results. " [But it never happened. So: "might have gotten results."]
Officially, the Times is on my side, as Philip Corbett explained in a recent After Deadline post:
A verb that is present tense in a direct quotation shifts to past tense in an indirect quotation after a past-tense verb: I am going to the store becomes He said he was going to the store, not He said he is going to the store. In such constructions, the future-tense “will” becomes “would” after a past-tense verb. In these cases, “would” is not acting as a conditional (He would go to the store if he needed something) but simply as the past-tense form of “will.”
Corbett calls this the "formal rule," but I don't think I learned it it as a formal usage; it's just the way everyone said it. So why the shift? It's another of those language mysteries. As I mentioned in a September Word column, Merriam-Webster's Dictionary of English Usage can't explain it, and doesn’t approve: 'We advise you to use might in all contexts where the past tense is appropriate or where a hypothetical or highly unlikely situation is being referred to.'"
But a more recent discussion at Language Log quotes the Cambridge Grammar of the English Language on the futility of resistance: "Conservative usage manuals tend to disapprove of [this] usage, but it is becoming increasingly common, and should probably be recognised as a variant within Standard English."
*It turned out the caption was wrong; the rapid bus service it referred to was not officially part of the Silver Line. But that doesn't affect the grammar question.