Final answer:
Rule 712.31 pertains to railroad safety concerning the minimum distance an On-Track Equipment (OTE) must maintain from a stationary train, which is not specified without the rulebook.
Step-by-step explanation:
The question refers to a specific rule (Rule 712.31) related to railroad operations, specifically, how close an On-Track Equipment (OTE) can be to a stationary train. The rule dictates that there should be a mandatory minimum distance maintained between an OTE and a stationary train to ensure safety on the tracks. Without the specific rulebook, it is not possible to provide the exact distance required, but typically, this distance would be set to prevent collisions, provide space for emergency personnel if needed, and enable sufficient reaction time for the moving OTE in case the stationary train starts moving unexpectedly.