Looks fine. I applaud the decision to use single track...as it makes a mainline seem much longer.
An shortline is also a wonderful idea as you can have a scale length train. How many layouts allow for a 70 car freight train? Not many. But with a shortline or narrowgauge...5-15 cars is a complete train.
You can also take advantage of those beautiful Spectrum 4-4-0s, 4-6-0s, 2-8-0s, and such.
One thing to point out on your trackwork: I think I see a kink and a rough vertical curve where you have the one line on roadbed and the other on plywood...with a connection in between. Otherwise it seems fine (I can't quite tell if you're using code 83 or code 100). The track plan looks good.
EDIT: So when does Thomas, Percy, and Gordon make their debut? I'm looking forward to getting my nephew Gordon and several of the mainline coaches.