Where would you find special instructions to change the retainer valve from Exhaust to High Pressure?

The Division Timetable is the most appropriate source for locating special instructions regarding the change of the retainer valve from Exhaust to High Pressure. The Division Timetable contains essential operating procedures, schedules, and specific rules for train operations in a given division. It is tailored to provide crucial information that impacts train handling and safety, including the operational conditions under which certain equipment adjustments must be made, such as the retainer valve settings.

In contrast, the other options tend to contain different types of information that are less specific to procedural changes of equipment like the retainer valve. For instance, the Engineer Manual is focused more on engineering standards and operational guidelines rather than specific operational adjustments. The Operations Handbook might cover general operational practices but is not typically used for detailed equipment-specific instructions. The Signal Protocol Guide, on the other hand, deals primarily with signal operations and protocols, which are unrelated to the mechanical adjustments of train equipment.