The announcement on the train went like this:
From Monday 20th of February, you will only be able to buy full-price tickets on board our trains.Fair enough. That says what they mean. Except it didn't! It said quite another thing, because they got the intonation totally wrong!
Sentences have intonation contours and prosodic units. It's where you put the stress and what bit of the sentence you treat as a phrase. Prosodic units line up with syntactic/semantic units, so the meaning is nice and easy to understand (I note that Wikipedia says they don't, though the examples there do, so I'm not sure what I'm missing).
I'm now going to try to convey the intonation with the use of bold text. The emboldened bits are meant to be given stress, and the bracketed bits are treated as a prosodic unit (the second one actually is a case where the prosodic unit doesn't match up with syntactic units):
You will only be able [to buy full-price tickets] on board our trains.Try saying them out loud. In the first one, the meaning is that the only kind of ticket you can buy is full priced. Correct. In the second one, though, the meaning is that the only place you can buy full-priced tickets is on the trains, not anywhere else. False! You can buy them at the station or online too! And it's the second one that was in the announcement on the trains. If I were more of a pedant I might test them on this principle. But I'm not.
You will only be able to buy [full-price tickets on board our trains].
I'm sure I've got all the technical stuff wrong here, but I tried the two intonations out on my friend Stuart and he had the same interpretation as me, so the point is right, at least.