To those worrying about lightweight spackle cracking...it doesn't happen. It doesn't shrink nearly as bad as plaster. Besides not shrinking, it stays slightly flexible, about the same as foam, so it will flex and give with the layout if things get upset. I have used it for home repair as well as model railroading for years. As mentioned by Dave Foxx, it dries to the consistancy of the high density foam we use for scenery.
A whole 4x8 done with only foam and light weight spackle. It was then transported from my house to the customers with no cracks or damage showing in the process.
I also use it in pre-tinted batches for roads either as a complete paving or a skim coat over a foam or styrene precut sheet. You can sand it afterwards to achieve varying amounts of wear and color shift in the road. Skimed over foam sheet, you can then flex it pretty subtancially and force it to crack to simulate older roads.
I even use the stuff for rock faces and carve in it as it starts to harden.