Electro Coupler, will this work?

Yes, it will work. You don't need a relay. You need a diode in series with the coupler coil, biased ("aimed") so that it conducts in the opposite direction from the horn circuit.  However, you can do this for only one coupler at a time, or else all the couplers so equipped will operate. In other words, if you wire up one loco this way, fine, but don't do it for all the locos that are on the track at the same time.

A more expensive way but totally selective, is to use a small radio control transmitter and receiver in every piece of rolling stock on which you wish to have this feature.   Try this:

http://www.ebay.com/itm/fixed-...;hash=item3a822a49d6

I have a small 4' x 8' layout, with 2 loops, so I would only have two trains running at the same time, and the loops although connected are electrically insulated from each other.  And I mostly just let the trains run. Only 3 sidings so not a lot of switching opportunities.

That rc module is interesting though. I'm not really knowledgeable about electronics, especially what's out there. But that has a number of possibilities. Thanks for that link.