What identifies all opposing regular trains that are due?

The A-Card is a document primarily used in LIRR operations that contains essential information regarding train movements, including schedules and statuses of trains. Specifically, it identifies all opposing regular trains that are due on a particular route or track. This allows train crews and dispatchers to maintain awareness of all active train movements in order to ensure safety and efficient operation.

Other options, while related to train operations, serve different purposes. The Form L typically pertains to special instructions or orders that assist in managing train movements under specific circumstances. Clearance Cards are used to verify the clearance of trains along the tracks and provide information about the physical space available. Blue Signals are employed to indicate that work is being performed on or near the tracks, ensuring that trains do not approach those areas. Thus, the A-Card stands out as the correct choice for identifying opposing regular trains due.