Fog machines generally use either mineral oil or a combination of water and propylene glycol. Ethylene glycol isn't used because it is toxic while propylene is commonly used in flavors.
Train liquid smoke material is generally a very light grade of mineral oil, sometimes very light. Different brands use different blends. Since I have access to the materials, I make my own. I make it dual purpose so that I can also use it as lubricating oil. I strongly recommend that you use commercial fluids meant for this purpose and not try to use something not designed as a smoke fluid for trains.
