On single track under rule 251, when can an extra train run without a train order?

Under rule 251, which governs operations on single track, an extra train is allowed to run without a train order specifically within yard limits, provided that there is a proper signal indication. This is vital for maintaining safety and communication on the rail system. Yard limits are defined areas where certain operating rules apply, and they typically allow for more flexibility in train movements, including the ability for trains to operate without formal orders as long as they are following signal indications.

The proper signal indication serves as a confirmation that it is safe for the train to proceed. This eliminates confusion and ensures that movements are coordinated effectively, preventing accidents. Therefore, under the conditions set by rule 251, the extra train's ability to operate without a train order is contingent on receiving clear signals that denote safe passage within the confines of yard limits.

In contrast, other options do not align with established procedures within the operating rules. For example, simply assessing safety or relying on a conductor's discretion lacks the structured oversight necessary for safe operations on a single track. Similarly, operating at any time when the signal is green could misinterpret the authority that is specifically tied to train orders and operational protocols.