Question

which must be installed halfway between the top of the guardrail and the work surface? top-rails mid-rails toe-boards wire mesh

Explanation:

Mid - rails are installed halfway between the top of the guardrail and the work surface. Top - rails are at the top of the guardrail. Toe - boards are at the bottom near the work surface. Wire mesh is a different type of barrier material and not specifically defined as being halfway in this context.

Answer:

Mid - rails