Is it correct? It's the message I see right before the metro/subway train shows up.
Shouldn't it be "train is approaching" or "train approaches"?
Either way this is just a fragment, so the question is what is the rest of the sentence? Your suggestions read as follows, to me:
I usually interpret it as the following:
There's clearly a lot of flexibility when you're only seeing a fragment of the sentence.
Yes, it's "correct". It is there to carry information, from the metro system to you, namely, that there is a train approaching. And that you should step back ;-)
In all this "proper grammar" and whatever, we (me included, see my rant-with-a-lot-of-comments ;-)) tend to forget that language serves a purpose, to wit, to carry information from A to B (where A to B are very often, but not always, individuals). And as long as it manages this task, to the satifsaction of, well, A and B, all's well.
In this case, I don't see any "failure to inform". So there, job well done, language! Good language, yes, good language! pats language on, uhm, head?
I would say it depends on where you are stood and the information being conveyed. If you are on the platform and you can see or hear a train then the message is providing specific information.
This also implies a connection, not always correct, to the next train arriving. To say
can be confusing, there may be several trains approaching, are you talking about a specific one or all trains?
Whereas if you were on the train the announcement would be
as this is specific to the train you are travelling in.