Learn German: Aufgepasst, der Zug fährt ab! - Watch out, the train is departing!

Using a past participle creates an impersonal, general command. This is typical for public announcements, warnings, or addressing a crowd where you are not talking to one specific person. It sounds more universal and a bit more abrupt than a standard imperative.

How does this impersonal command differ in feeling from using an infinitive command, like aufpassen?

Both the infinitive and the past participle can be used for impersonal commands. However, the past participle (aufgepasst) often feels much more immediate and urgent, making it perfect for sudden warnings. An infinitive command sounds slightly more like a general, ongoing instruction.

Since aufgepasst comes from the verb aufpassen, how does it differ from just shouting the noun Achtung?

Both are perfectly natural ways to say Watch out! or Attention! However, Achtung is a noun that simply flags a hazard, while aufgepasst is a verb form that actively commands the listeners to do something (to pay attention).

A quick refresher: why is the word ab at the very end of the sentence?

The verb for to depart is abfahren, which is a separable verb. In a standard main clause, the conjugated stem (fährt) takes the second position, and the prefix (ab) gets pushed to the very end of the clause.