How long are timetable schedules effective?

The correct choice states that timetable schedules are effective for 12 hours after their specified time at each station. This means that after the time a train is scheduled to arrive or depart, the timetable remains in effect for a period of 12 hours. This provision allows for operational consistency and ensures that any alterations or operational changes can be handled within a specific framework of time, promoting safety and efficiency in train operations.

The rationale behind not choosing the other options lies in the operational standards set by the LIRR. Schedules are not simply valid until a new schedule is created; they maintain their relevance within the defined timeframe. A 24-hour expiration would be unnecessarily long and could lead to confusion or operational inefficiencies, whereas the validity of a schedule until a general notice is issued doesn't account for the specific limitations that need to be in place for clear and effective train management. Thus, the definition of effective time in the answer reflects practical considerations in railway operations.