AHaha Anna, my first ever Christmas gift from my father was a model train, the latest one he needed for his display! I was 7 weeks old...

Hydrocal, Plaster of Paris, and other gypsum cements, and for that matter lime cements like Portland cement, all harden by absorbing water, and are therefore completely waterproof (although they can certainly be eroded by sustained water action) when fully set and dry. They can be internally colored with any non-reactive pigment (many model railroaders throw in some burnt umber, so that if the texturing layers are chipped off, there isn't a stark white spot), stained with dyes, and painted with any kind of paint you can come up with. And of course, it doesn't decay or burn, and there's not much that can eat it.

Royal Icing, on the other hand, is neither waterproof, nor decay-proof, nor fireproof, nor vermin-proof. And neither is buttercream.

Does Royal icing using glycerin get hard that you can sand it after the first coating? Is it better without glycerin?

Glycerol (the actual chemical name, or in the strictest Geneva nomenclature, "1, 2, 3 propantriol" or "propane-1,2,3 triol," CH2OHCHOHCH2OH or C3H8O3) is very hygroscopic. It draws moisture from the air. That's what makes it useful in moisturizing lotions.

I should think that in royal icing, it would prevent hardening.

Hi:

So, you said covering with buttercream can work?  I was going to put some cream of TARTAR to make it harden what do you think?

I don't see cream of tartar doing much good. 

A basic shortening based BC--shortening, powdered sugar, and enough water to make it work properly--should be fine.  It'll air dry pretty quickly because it doesn't need to be very thick to cover smooth styrofoam.  It can't be handled roughly, but should hold up well with reasonable care.

AI use JOINT COMPOUND. My husband is a general contractor, I noticed the way it dried when he was drywalling a house, tried it tinted with acrylic paint. It worked beautifully, texture very similar to BC. Fondant, gumpaste and royal icing adhere to it really well. Home Depot 17 pounds for $8.89.