I was looking around our local Ace Hardware for some dry oxalic acid when I discovered this product in a one quart bottle for less than $7. It's already dissolved in water, and claims to be concentrated so that it can be diluted to make 5 quarts of solution for bleaching and cleaning wood. I had an empty one gallon covered container handy, so I diluted a quart of this concentrate to make about 3 quarts of solution.

It does a good job of softening train smoke overnight, so that the gunk can be removed with a gentle application of a scrubbing pad, with some Barkeeper's Friend for tough spots. In the past, I have used lye solution for removing train smoke and other organic goo, and hydrochloric acid to remove cemented pins. Both will clean glass very well, but are more hazardous to have around the house. This oxalic acid product seems to be just fine for general cleaning, and certainly is easier to use.

For a good general article on cleaning insulators see http://www.insulators.info/care/cleaning.htm.