That's the most plausible explanation I've heard so far.

Comparing the load to the flatcar says these things are about 4', maybe 4'-6" max, in diameter.  That's a reasonable size for water piping.  Two to a car longitudinally says they're 20' long more or less, which was and is a common length for joints of pipe.

I found a 1942 catalog of wood pipe which claimed extensive use, even at that date.  The war probably boosted its use a bit.   http://www.waterworkshistory.us/tech/Pipe/1942WoodPipe.pdf
Not incidentally, they also built water tanks, which if I recall correctly had some use on railroads when they used those external combustion locomotives.

I also ran across this photo -- http://www.waterworkshistory.us/tech/Pipe/1942WoodPipe.pdf

Ron Merrick, piping engineer