Does the word 'ensure' imply a requirement to act?

Solution 1:

Well, normally ensure does entail a responsibility to correct something. However, in your example, there is nothing that can be done but to wait until the door is alongside the platform.

So I think it would be more accurate to say "check that" (as you mentioned), or confirm [that], or verify [that]

Solution 2:

"Ensure" has several senses of obligation, contractual and otherwise, but it also has the meaning of assure, i.e, to convince yourself of the state of something. Considering that passengers have no means of correcting the train's position at the station, any obligation they have is to refrain from stepping off the train if doing so means they will fall to the tracks.

Solution 3:

I think there IS a requirement to act.

Had the sign simply said Please ensure this door is alongside the platform then it would have been incumbent upon you to get out and help push the train in the event it wasn't.

But since it is qualified by ...before alighting, you can meet the requirement (should the door not be alongside the platform) by not alighting.

But it was a good question and merits an up vote.

Solution 4:

Ensure is seldom used to imply obligation to act in a context where your responsibility and capability to act aren't already clear.

For example, if your boss says "Ensure your TPS reports have the new cover sheet," he's not just asking you to look at your TPS report and shrug if it has the old cover sheet. He's clearly telling you to fix that scenario by replacing the old cover sheet with the new one.

On the other hand, if a mother tells her daughter, "When you cross the street, ensure no cars are approaching," she's not telling her daughter to somehow stop cars that are approaching (this would require superpowers or obnoxious arm-waving). She's simply telling her daughter to verify that the street is safe to cross, and if necessary to wait until it is safe.

So ensure simply means "do not proceed until some requirement is met." If the context makes it clear that you're responsible for and capable of getting those requirements met, then it does also imply a command to act in that regard.