With regard to deck planks, my cars following the instructions on the Gnine.info site had 5/32" wide planks, 5" wide in 1:32n1. Those would be about 3-1/2" in G scale. Most important is to select a width that gives an even number of boards for the length of the car; I tweaked the width a bit to fit, so the width varies a little from plank to plank.

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Start I made some years ago on a 1:72n2 loco using a Bachmann Plymouth. Deck plate is probably a little narrower than one would want for G9 (and was cut a bit oversize in length), but shows how the hood/bonnet section is low enough to easily fit a seat over it, while the cab section can be hidden under the G9 hood/bonnet. Engineer is from a 1:24n16" loco, so would need to be posed with his feet closer in for a 7.5" gauge loco.

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