Cast iron sleeper types: Which designations refer to cast iron (CI) sleepers used in permanent way practice?

Introduction / Context:
Cast iron sleepers were historically adopted on many networks due to durability and suitability for certain traffic categories. Knowing the common CI sleeper forms helps in identification, maintenance planning, and understanding legacy designs in yards and secondary lines.

Given Data / Assumptions:

• CI sleeper families include multiple shapes developed for different gauges and fastening systems.
• Terminology such as pot, plate, box, and duplex corresponds to design variants.

Concept / Approach:
All four listed names denote common types of CI sleepers. Pot sleepers feature bowl-like castings; plate sleepers have broad plates supporting the rail; box sleepers are hollow box-type castings; duplex sleepers are paired or twin units used especially on MG in certain applications.

Step-by-Step Solution:
1) Identify each term as a recognized CI sleeper type.2) Confirm none of the items contradicts the CI material or functional application.3) Select the inclusive choice.

Verification / Alternative check:
Permanent way manuals catalog these CI types with drawings for rail seat, fastenings, and ballast contact surfaces.

Why Other Options Are Wrong:

• Choosing any single type ignores the existence of the others; all listed are valid CI sleeper forms.

Common Pitfalls:

• Confusing CI sleeper forms with timber or concrete sleeper variants.

Final Answer:
All of the above.