If it is coming from the sleepers your only way forward may be to replace them. I had two wooden half barrels that had large plants in them. One of them suddenly developed a type of mushroom fungus. I tried various treatments but eventually just had to get rid of it as it kept coming back. Funnily enough it was the one in the sun not the one in the shade, you would sort of expect it to be the other way round.
I have no idea.. I've seen it before on rotten wood in the forest, etc.. but never taking over plants. Personally, I would be tempted to use something like a liquid copper fungicide in a few select areas (not your entire garden, yet) and see how that works out. I've never used it before or know if it will work.. but it sounds like you've tried lots of other things already.