Yes, there are rules, and recommended practices based on the amount of traffic. That un-signed crossing is a liability lawyer's w-- dream. It is the RRs responsibility to assure the crossing is properly marked, and given the quality of the grading and the obvious maintenance of the RR ROW it is clear the RR knows about the crossing (versus a farmer "rolling his own" without notification).
@Point353 's stop sign example is good example of an RP for the lowest levels of private traffic, but given that the picture appears to be of a small commercial elevator with public access, passive crossbucks with a stop or yield sign would be in order.
I tried to find the rules in an online
MUTCD, but it's all engineer-speak so a clear and easy reference for this scenario was nowhere to be found.