If the triggering rail spanned both approaches to the crossing, wouldn't it work in both directions....assuming your layout is large enough to support a FULL train length on each side?
To your point, you do not want a situation where the Yellow and White wires are simultaneously triggered! I have dissected the electronics of the MTH crossing gate and it does not take kindly to such a proposition.
Above diagram illustrates another "concept" in isolated-rail triggering. More than 1 isolated-sections can be tied together to create a single trigger wire to the gates. In this case there are 2 sections for the gates-down (yellow) trigger and 2 sections for the gates-up (white) trigger.
The MTH crossing gates are kind of a special case because it will flash the alternating red lights whenever the gates are away from the upper-most position. It will do this without a trigger on either input. So in addition to the FULL train length requirement between down and up trigger sections, the trigger sections must be long enough so a passing train triggers the gates to go all the way down or return all the way up. It only takes 2-3 seconds for the gates to change positions so this should not be a problem.
In both diagrams above just the 2 trigger wires (Yellow and White) are shown. The other 3 wires are connected per instructions.
Thanks Stan, this is what I was looking for.