Lionel TMCC board look corroded ..what should I do...

Picture would help.  Clean it with either contact cleaner or Isopropanol.  To protect it from future issues I would Humiseal the board being careful not to get the coating in/on the connectors/pins, contacts or what have you.

As Gary mentioned above, pics would help.

But if I had to guess, you're seeing the "fog" on the underside of the RailSounds board. It's not corrosion, but rather residue from the soldering process, as you'll notice it's only found around the through-hole components and not ones that are surface-mounted.

A light scrubbing with mineral spirits or contact cleaner and a soft toothbrush will clean-up the majority of it. But it's really not hurting anything.

TRW

I actually had this happen on my 1996 TMCC Santa Fe B unit.    At one point it was packed away for 7 years.The sound began getting finicky.

A simple cleaning with a toothbrush and contact cleaner (as mentioned above by Superwarp and TRW) did the trick.

I use 99% Isopropyl Alcohol to clean boards, and it works fine on the white stuff as well as the flux left over after building a board.

DanssuperO posted:

I opened my newly purchased   lionel TMCC  diesel made in @2000 ( wow 19yrs old!!) ... it works perfectly .. I wanted to make sure I lube it  etc ... When I took the shell off I noticed a lot of  white gray  what looks like corrosion ..

Should I be concerned or leave it alone  ..  thanks Daniel

"it works perfectly." "...leave it alone?"

Self-answering, I'd say - though the soft toothbrush cleaning - no solvents, I'd say - is a good idea. Boards grow "hairs" from the solder over the years. Almost always benign, but some do occasionally transmit a wee current to unintended places, causing issues. 

I have done literally thousands of boards with the Alcohol, it's never been an issue with any of them.

D500 posted:

"it works perfectly." "...leave it alone?"

Self-answering, I'd say - though the soft toothbrush cleaning - no solvents, I'd say - is a good idea. Boards grow "hairs" from the solder over the years. Almost always benign, but some do occasionally transmit a wee current to unintended places, causing issues. 

Tin Whiskers can be a pretty serious problem for electronics.  (more so for the type that end up in use somewhere where servicing is not possible - note the web page is from NASA )

-Dave