I'm stumped. Why would there be no lumber on flat cars in Oct-Dec of 1951? Was this the time period after the Streamliner accident (a piece of lumber shifted on a flat car, and broke the windows of a passing Streamliner, showering the passengers with broken glass) ?
I cannot speak for other railroads however on the NP, after lumber on an open car shifted and took down a signal, lumber loads on open cars both flats and gons, were handled on a separate train east. Traveling at a 35 mph restriction until diesels took over and then it was granted a 50 mph speed limit. I want to say it was the J manifest. And yes, oscillations caused by the steam locomotives carried through the train were thought to be the cause.
So if you looked at NP trains headed east, you would find the same phenomenon, flats vs XM's carrying lumber.