My second level mainline loop is flex track, roughly O90 diameter. My 3.5" from center track clearance is enough for all my locomotives, at least all that I've tried. I've run most of the big articulated locomotives as well as the aforementioned C&O Coal Turbine, and they all made it around without clipping the post at 3.5". I really don't anticipate any larger locomotives showing up that would challenge that post. 
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