Ahhh ... so the PVC caps have markings on them? That is good to know While I doubt that we have a sewer clean out there, as it is on railroad property and at least a 1/2 mile from the nearest house, I do not doubt that it may be something else. However, since finding this, I have noticed more of these types of enclosures around railroad tracks in my area ... and there are alot of benchmarks in my area that appear to be on railroad track property.
Rusty0101 ... thanks for the tip, I'll have to return to this one when the weather warms to get to the bottom of what it actually is.
Thanks again!