The minimum length of fixed polarity track between two AR sections needs to be only as long as a typical power pickup truck where more than one axle and steel wheels are connected electricity together. (Think diesel truck or steam engine drivers)
Then as that truck spans the gap from the “leaving” AR #1 and enters fixed polarity track it will cause AR-1 to flip to agree with the fixed polarity.
Then as that same truck spans the gap at the other end of the fixed polarity section into AR-2, AR-2 will also be caused to flip to match the fixed polarity. So then AR-1 = fixed = AR-2. DONE
So how long does the fixed polarity track section need to be ? Only as long as the multi-axle power pickup truck. Any additional length is unnecessary.