While I’m am getting advise from the experts, I need to do several dirt floors. Again these are for indoor, portable\transportable buildings. I have a blacksmith shop and a foundry which have dirt floors (more of a sandy consist as spots), as well as a car shop where the dirt is more or less covering the ties (assumes there are or were ties under the rails). Any thoughts?
Use real dirt held on with white glue?
I agree with Bart. Nothing looks like dirt like dirt. 
Strain it through a screen first to remove odd chunks and debris.
Alot of people also bake it on an old cookie sheet to dry it out and kill any left over “stuff” that may be living in it.
Terry
Follow Richard and Terry’s advice, but get a small toaster oven second hand at a Goodwill or Salvation Army to do the baking, you can do small batches as needed and it doesn’t make your wife go bat-guano crazy to find her favorite cookie tray full of stinky dirt baking in the oven (not me, someone I knew 
) One like this:
(http://sp.life123.com/bm.pix/toaster-oven.s600x600.jpg)
The most important reason to do the baking is A: kill any critters and no-seeums, B: Kill any organics like mold spores, and C: dry all the moisture out of the soil to dry it out. This makes ordinary dirt a much easier modelling tool.
thanks for the feedback.
I think the wife will be impressed with my new cooking abilities…
Perhaps the outdoor gas grill might make an appropriate cooking vessel… In my case, it’s so handy to the train layout it seems irresistible!
You can go to a Home Depot or Lowes and get the Polymeric Sand, which is already sifted, add water, let dry, and DONE, don’t like the result? add water wipe off sand, try again. Many different colors to choose from if your near a good Hardscape supplier also
