Rather than plan so far, I've taken a general concept, and built as I went. Few plans ever go unchanged
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I have yet to tackle the "big one". When I do, I think it will be a lop sided dog bone. Two main line loops hugging a wall and looping in the corners , 4 block control, and two cities. Later lines, & cities built off that later
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No concept has remained totally unchanged for me, except a perfect 0-27 circle 
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While I have three to keep me busy now, any long periods without a layout, had a carpet central to keep me into it.
That's how a perfect circle ended up on a small table here. I moved it to vacuume, and left it there. At Christmas it got a green table cloth & a tiny tree. Then in Feb., I fasten the cloth & left it for grass 
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