A slight correction to Herbert's comments above. In fact, signal boxes not only had Läutebuden, the green 'postboxes' shown above which were outside and signalled train movements for their own tracks, but also Turmläutewerke literally tower bells i.e. small wall-mounted bells inside the signal tower for line(s) at the other end of the station.

Smaller stations, where there was just a ground frame or signal cabin attached to the building, do not appear to have had Läutebuden at all. Instead there was a wall-mounted bell in a box, the Bahnsteigläutewerk (platform bell). This black rectangular box had a white disc on the side and a weight shaped like a Knackwurst dangling below it.

Presumably there were no bells at unmanned stops or halts, but I'm not sure if the typical branch line Agentgebäude leased by 'railway agents' had anything either.

Paul, of (formerly Prussian-occupied) Lower Saxony