Sending and receiving addressable contact closures.

["Alan Ainslie" <alan.ainslie@xxxxxxxxxxx>]

Some clever guy on our group must know the answer to this.  I have been
looking in all the wrong places I guess.:-

I need to send instructions to a number of switches to close or open:
accessed by address.

I also need to read the status of other switches as inputs.

This is all ON/OFF so no proportionality and I do not want to use a PC, but
am happy to make up a central controller and then to make up the devices
that go onto the control bus.  Ideally control bus would be two wires plus
the heavy 12V supply to power relays and things.

Anyone any ideas for chip set to do this robustly and economically. 
Devices
will be outdoors.  Everything is 12V only.  Numbers?  Less than 50 of the
addressable switches and less than 20 of the input contacts closures.

Application?  Well, almost an automated home: in truth this is to control
signals and to read status of various devices on a 7 1/4 gauge large steam
railway located-----in an automated home.

Any railway nuts out there welcome to a day on the Spinney light Railway in
Surrey.  Fully signalled at present using authentic 1930s railway
technology.  new stuff is for a large extension and I really don't want to
be running more miles of cables for the mice to eat.

Alan