A portion of single main track between yard limit boards, as specified by special instruction.

• A. Yard Limits
• B. Siding
• C. Two or More Tracks
• D. Regular Train

Answer: (A)

The main idea being tested is yard limits: a defined stretch on the main track bounded by yard limit boards where special operating rules apply. The phrase describes a portion of single main track between those yard limit boards, with the exact extent and restrictions set by a special instruction. This is the area where yard procedures govern movements, speeds, and protections, distinct from regular mainline rules.

Siding refers to a separate track for parking or passing, not a segment of main track. Two or more tracks points to the number of tracks, not the defined yard-limit section. Regular train describes the type of service, not a track designation.