Cold weather sometimes has interesting side effects -- and exposes problems that were there all along. Recently, the big yard in my layout developed a short. The PM42 triggered, of course, and my control panel showed that the problem was in the Deseronto yard area. I took everything off the track and started poking around. Nothing... checked all the turnouts, trackwork, nothing showed. Interesting thing, though. When I measured the resistance between the rails I got 84 ohms... an odd number. FInally, by isolating chunks of track and walking the line with my ohm meter -- I found it. A custom crossing had some carbon down in the frog gaps.  

I inspected the gaps with my microscope and found that several of the gaps had blotches of carbon down between the rail ends. As the layout chilled down with the changing season the gap likely closed when the track got cold enough -- just enough to violate the gap and induce a short. A few minutes work with sanding film and the problem was fixed. After a couple of days trying to find it...

greg