The public transport service in Hannover/Germany once had a screensaver that you could configure to show the next departure from your nearest station. I thought that was clever marketing. Today you probably could implement this as a web service.
That train schedule seems like a cool idea in and of itself though.
There may have been some special interface that the program being run was expected to conform to so the screensaver subsystem would invoke it, but (IIRC) a screensaver could do anything an ordinary program could do. (That was the big reason for being cautious about where/who you got your screensavers from.)