Machines are running and then they stop - sie halten an. So, for individual means of transport or machines the meaning is always "to stop": der Zug/das Auto/der Bus etc. hält an/hielt an/hat angehalten (the train/car/bus stops/stopped). Traffic and rain are flowing (sie fließen) and when they do that continuously: sie halten an. "anhaltend starke Regenfälle in Sachsen" (it will keep raining heavily in Saxonia) or "anhaltend starker Verkehr auf der A8" (heavy traffic on the A8). As you can see, in the meaning of "to continue", the verb is often used in the Partizip I, but you might also hear "der Regen soll noch über's Wochenende anhalten" (they say it'll be raining until Sunday/next week)

You could actually stop a car - ein Auto anhalten - while it keeps raining - während es anhaltend regnet. You could even hold a measuring device - ein Messgerät anhalten- so I could read what it says. :)