When is the brake system of a train recharged?

The correct choice is when the ME-23 brake valve is in release, as this is the condition that allows the charging of the brake system to occur. In this position, the brake pipe is pressurized, and air is allowed to flow into the brake system, replenishing the air needed for proper function.

In contrast, if the ME-23 brake valve is in an emergency position, the system is designed to deplete air quickly to apply the brakes as rapidly as possible, which would not allow for recharging. Similarly, when it is in a lap position, the flow of air is blocked to maintain the current state of the system rather than recharge it. When the valve is set to service, the brakes are applied, which again does not facilitate recharging the system as it is focused on applying braking force rather than allowing airflow for recharging. Thus, only the release position effectively enables recharging of the brake system, making it the correct answer.