Two thoughts:
4.25 C/L may be a good sized regret down the road. That limits loco size for passing long cars.
PR facilities great idea however it would be best outside of the 12 X 2? foot print. In reality a 12' width is a marginal bet with sweeping curves to start with. To cut in a PR brings overall design and significant operating compromise.
Remember, this is your retirement dream. You have worked, saved, spent, planed, then thrown in the great PR idea only recently. Consider a bump out in the design of the room. An ADA bump out. None of us has any guarantee, you may well be the one needing the turn around space. Having spent some time in a chair post surgery I made my last PR ADA compliant.
If the structure was 5' wider you could still have a center entrance with a PR to one side and a utility room to the other side. You will greatly appreciate both provisions.
I vote for a pair of 110 CFM fans, one big stinker could send everyone out into the cold. (it's so easy to spend other peoples money. 
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