Was working on a GP9 I received and noticed the following on the control board under the body. See pics. What is it? Looks like battery corrosion but that doesn't appear possible. Any help identifying? Thanks!
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Mold or tin(Sn) whiskers. Check with a microscope.
Thanks. Doesn't scrape off. I would think mold would. Do the wiskers pose a problem or could they damage the board?
Whiskers have taken out space probes and nuclear power plants... so yes, they can cause shorts. The reduction of lead in soldering alloys has caused this in many places.
Thanks Rob. Any idea what it costs to replace these boards?
No idea - you can search the part number on the Lionel Support website.
It only appears around the solder for through-hole components, nothing surface mount, so I lean towards it being a remnant of the manufacturing process (cleaner? flux?) or perhaps it is some sort of mold as Rob suggested above.
I do have some of these RailSounds 2.5 boards that haven't ever been used, and they exhibit the same phenomena. I've tested them all, and haven't seen any issues what-so-ever.
If you need to replace your board, I doubt this "fog" is the reason for the failure. If the board works fine, I can't think of a good reason to replace it unless it fails. And if you need any of these boards, shoot me a note (address in my profile) as I have some for sale.
TRW
Clean it off with Isopropyl Alcohol, I use 99%, but the 91% will works just as well. I've cleaned a number like this, I use an old toothbrush.
No, I don't use the toothbrush for it's intended purpose after cleaning circuit boards with it! 
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