I recall a thread where the idea was to have a turnout alternate direction each time a train triggered the insulated section ahead of the turnout. So train goes straight on loop 1, turns out on loop 2, goes straight on loop 3, etc.
In a recent thread a guy wanted a brief relay pulse (1 second or so) to drive a semaphore when the train entered an insulated section, and then another brief relay pulse to drive the semaphore when the train left the section.
Another source of ideas will be the existing signaling manufacturers and their smart boards. AFAIK the semaphore application above ended using an off-the-shelf Azatrax detector/signaling board.
While brainstorming is useful, I think the sky's-the-limit, kitchen-sink approach can bog down. So while I am a proponent of clearly identifying the what before choosing the how, if the what is so open-ended as to be ill-defined then maybe the hammer-nail approach deserves a look. To that end, in GRJ's thread about his insulated-rail based relay he alludes to some day putting in some smarts. Take a look at so-called DCC-ready engines. To add smarts to such an engine you simply pull out what amounts to a "shorting plug" and install the DCC-electronics which now adds command-control of motor, lights, etc.
So suppose GRJ's relay board...or something similar had a "shorting plug". With the plug in place it behaves as originally intended with the occupancy detection directly driving the relay. Now pull out the shorting plug and install a short cable to your Arduino module. Now the occupancy detection is routed to the $2 Arduino and you can add smarts to the relay control such as firing the relay on every 20th trigger. So the Arduino is intercepting the occupancy detector, doing its thing, and then sending back the relay control. I believe you can imagine a situation where that cable even carries power to the Arduino (it being a low-power digital device).
So how about a more complicated situation with 2 detector sections driving 1 relay (aforementioned bi-directional idea)...or 1 detector driving 2 relays (aforementioned semaphore idea)? Seems the same single Arduino can handle those situations with 2 identical cables (one from each detector/relay module).
I'd think programming or re-programming the Arduino should be something akin to downloading MP3 files or the like. Someone else writes the sketch, you download it from OGR or wherever, you plug the Arduino into USB to get it, plug the cable(s) into the detector/relay module(s) and off you go...
