Dish washing detergent + water. Costs almost nothing. 4 drops of Dawn (it's concentrated) per quart of water. But my water is somewhat hard, soft water may get by with a drop or two less of detergent. Or you may need a drop or two or more if using a non-concentrated dish detergent.
A 50/50 mix of Elmer's white glue and wet water is my formula for ballasting. Elmer's is NOT water proof when dry. It can be re-softened with water after it is dry. Makes it easy soften ballast down the line should you decide to tear up track and re-use for another layout.
A fair amount of yellow wood glues are also not totally water-proof when dry. They too can be softened with applications of water, but may take longer to soften than white glue.
I am under the impression that Mod Podge (and any other artist's matte and gloss mediums) ARE waterproof when dry. You can use them the same as you would Elmer's glue, but keep in mind they are water proof when dry. You might have to jack hammer the stuff off if you ever decide to tear up your track for another layout. Regardless, I use Mod Podge Matte for tree building, and Gloss for simulating water. That way, future applications of wet water for ballasting and general scenery work will keep the leaves (ground foam) on the trees and the water surface from melting.
To sum it up, you may want to consider what you want to do a year or two or three after you have glued something. My general rule of thumb for dried glue is............... White glues: water soluble. Yellow glues: water resistant. Artists acrylics and mediums: water-proof.