One is XTrkCad, http://www.XtrkCad.org it is free.
Do the demo/tutorial before you try to use it. Then work with it for a while, then do the tutorial again.
It won't design for you, but it does get the layout on paper!
Jeff But it's a dry heat!
RTS 7.0 avaliable from Atlas is a pretty good freeware program. Not too complicated but I'm sure there are other programs with more bells and whistles that might be more suitable if your looking for an in-depth program.
-beegle55
de N2MPU Jack
Proud NRA Life Member and supporter of the 2nd. Amendment
God, guns, and rock and roll!
Modeling the NYC/NYNH&H in HO and CPRail/D&H in N
While not specificaly for model railroad use Google SketchUp is a free 3D program that is very easy to learn how to use, and will let you build your layout on the computer, before, you start cutting any wood, and will let you see how it will go together layer by layer.
http://sketchup.google.com/product_suf.html
It's so easy to use, it can be adictive - once I got it, I spent 2 days doing nothing but just drawing things with it.