Not trying to start an argument, but…
Several data points…
The Du Pont Historical Society web page about the project says the line (or at least the preserved equipment) is 36” gauge
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www.dupontmuseum.com]
A quick search of Plymouth records for 1941-44 show two locomotives used on the line, c/n 4119, built 12/1944 an FLH-2, originally used at Louries Colorado, later at Du Pont WA, and c/n 4352, built 3/1942, model JLA-2, built for Du Pont, for use at Hanford WA for US Atomic Energy, to Du Pont, Du Pont WA, to Weyerhaeuser, to display at Du Pont… (the preserved locomotive)
Randy