Ward,
For a conventional (non-command) approach in 'O' Gauge you can certainly do an A.C. equivalent to your diode setup. You would use strings of bridge rectifiers to drop the voltage as the station is approached, instead of strings of individual diodes.
There are several auto-reversing units available, like the DZ-1240 Auto Stop & Reverse Module from Z-Stuff. Generally these use an isolated outside rail to detect that the RDC is approaching, and then is at, the end of track.
As far as I know to initiate horn and bell sounds at the correct positions some of these auto-reversing systems may support this directly, but for the most part this needs to be home-grown. A D.C. offset voltage must be switched into the track power feed, in one direction to blow the horn, and in the other to start the bell ringing. This could be done in hardware with relay logic, or in software with an Arduino with a relay on the output, or similar.
Perhaps someone else knows of 'O' Gauge programmable horn/bell actuating systems?
Mike
