Well thats a little bit complicated! Well in the path_corner's atributes you will see a 'fire on path' value! In that value write the name of a multisource,when the train passes that path_corner the multisource will be triggered! In the button's master value write the name of that multisource. Now when the multisource is on the button can be activated and when it's off it can't!
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Do the same thing with the other path_corner and make sure that only one object targets a multisource or else the multisource will only turn on if all of them are on!
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There is another way to disable buttons and I was writing a tutorial on it but I stoped cause of the compotition! I think I'll resume writing it now!
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