40 turnouts = (at least) 4 AIUs. If minimizing wiring is an objective, several OGR threads discuss using longer TIU-AIU and AIU-AIU cables than the ones supplied. So rather than a set of centralized AIUs next to the TIU, you can run longer versions of that snap-in "phone" cable to place an AIU closer to the cluster of turnouts it controls. Shouldn't be hard to dig up the OGR threads including where to buy the cables.
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