If the engineer requests that retaining valves be set, but no quantity is specified, how many must be set?

When an engineer requests that retaining valves be set without specifying a quantity, the protocol is to set all retaining valves. This approach ensures optimal braking performance and safety for the train, as retaining valves are used to maintain brake pressure in the train's brake system during various scenarios, such as when descending grades. Setting all retaining valves provides maximum control and stability of the train, particularly under conditions where additional braking might be necessary.

Choosing to set none, one, or only half of the retaining valves could lead to insufficient braking force or uneven braking across the train, which can compromise safety and efficiency during operation. Hence, setting all the retaining valves is the most prudent decision when no specific instruction is provided by the engineer.