This thread is interesting. I can only find reference to concrete as an alternative in the UP Common Standards for RR property boundary posts. There is reference to Santa Fe experimenting with concrete signage in the Southwest in the 1915-21 period. That would have been the heyday of the SP/SF partnership with NWP. Although substantial, cross bucks were probably hard to maintain once damaged. They were reinforced with heavy gauge wire. It would take something more than a Fairmont Crane to repair one. Somewhere out there in research or someone's collection is a bulletin or article concerning concrete signage as someone's railroad practice?
ARD