Otto sent me several photos of U.P. helpers on Cajon Pass with cabooses in front of the helper. One photo distinctly shows two steel CA-3/4's and a wooden CA-1 (modified in the LA Shops with a high cupola) being shoved along with them.
Of course, the CA-1's had steel underframes as opposed to the Common Standard CA's, which had steel reinforced wooden frames.
However, U.P. shopped a few CA-1's to specifically refit them with even stronger frames as indicated by Don Strack in his Utah Rails website. The CA's were almost all shopped to refit them with stronger steel frames which is why the last wooden caboose on the U.P. lasted into the '70's. Good cars.
That particular CA-1 with the extra high cupola is well documented as being rebuilt by the LA Shops from a slightly wrecked non-typical CA-1 (CA body on CA-1 steel underframe)...so it gets "more complicateder" as I get deeper into it.
I agree that as a general practice, wooden cabooses were not shoved. The U.P., on the other hand, seemed to shove them on Cajon Pass, but never never shoved any cabooses on the Wasatch Grade, either wood or steel....and I don't see any rhyme nor reason to it, unless there was a formula for train tonnage and grade percentage that mandated cabooses be placed a certain way on different grades in differently trafficked areas of the road. But, that is just conjecture on my part.
Maybe Don knows the answer. If so, I'll post it here.