Allen
First of all congratulations for putting your plan to paper. Conceptually it seems like an interesting plan, if it invovles say 2 different railroads.
The size of your roundhouse seems a bit small to me, as do the lengths of the tracks around it. Are you planning on using 18" radius curves? That's about what they look like to me, judging by the scale you're using. From what I can tell, the area you are planning the layout in, is about 12' x 17'-is that right?
The trains will operate more reliably, + look better, if you increase the radius to say 22-26." I assume you're working in HO scale. Your sidings aren't going to be very long, based on what's on the plan.
Bottom line is if you like what you've got, that's all that matters. Most people build several layouts over time, which isn't an issue, if you learn things from the experience.
Carl