The Gateway NMRA Project Railroads website has a few layouts that fit within the parameters of 4x6 in HO with diiferent themes. Check them out. http://www.gatewaynmra.org/
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Mike Kieran
Port Able Railway
I just do what the majority of the voices in my head vote on.
Here's another option, this is 3x5 but could easily be done in 4x6 by using all 18"R curves, this uses 15"R curves at the oval ends to save space.
At this small a size go straight DC, use Atlas controls, its a little more complicated than DCC but it teaches you basic electronics and gives you hands on experience with wiring, something every 11 year old should know ;) . If you already have a DCC system then go that route as its the start-up costs that are usually the highest. Also check out BlueRail which uses a smartphone as the controller, saving big $$.
(edit) NEVERMIND, the original thread is 10 years old, this kids already in college chasing girls and weed.
Have fun with your trains