What is the maximum permissible distance from the bottom tread of a side ladder to the top of the sill step?

The correct answer identifies that the maximum permissible distance from the bottom tread of a side ladder to the top of the sill step is set at 21 inches. This specification is crucial for ensuring safety and compliance with regulations related to ladder use and access equipment on rail cars.

Maintaining this distance helps promote safe transportation of personnel by minimizing the risk of slips or falls while climbing onto or descending from rail equipment. A distance of 21 inches provides enough clearance that aligns with ergonomic practices, thereby facilitating smoother transitions and reducing the overall risk of injury during operational tasks.

The other options list distances that do not comply with established safety regulations. Distances that are too close may pose a safety hazard, while distances that are too far can impede quick access needed during emergencies or normal operations. Hence, adhering to the outlined standard of 21 inches is essential for maintaining safety standards in rail mechanics.