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Here's my final schematic. I've only put one signal and point motor in - duplicate to taste. Multiple MCP23S17s can be wired in parallel, provided each has a unique binary value on the A0/1/2 pins.

Diodes D1/D2 should be something like a 1N4004. I read somewhere (must find the link) that the inductive spike could be 300V, so make sure they can handle that.

Edit: The image is now lost to time.