Roberto,
Another option would be to use a DPDT 12vdc relay to switch the polarity on the two frogs that must be switched and have a detection circuit on either side of the diamond that will pull in the relay as a train approaches from that direction. The other leg can be wired such that when the relay is not energized the frogs have the correct polarity. This is all outside of JMRI but is pretty simple and does not depend on any external connections. I have successfully done this and it does work.
Mark Juett
Ace Services LLC
Your First Choice
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