Survey Monument Installation 

just like train ballast, there is more surface area making it very stable where as a block of concrete is easier to shift

Sometimes they have to be. Also, here it is very accessible and protected.

lol

"It's good enough. Let's go to lunch!" They said.

The plumb bob is centered on the cross ties that located where the monument needed to be set. The actual point needs to be carefully located. Th construction need not be so precise.

Those would not be centerline monuments, then. That's what these are.