Thanks for reading my article, and I'm glad it stirred some interest.
On some parts of the Santa Fe's Los Angeles Division the use of retainers was at the conductor's discretion, but their use on westward freight trains departing Summit was spelled out by special rule in the employee timetable. The number of retainers used was dependent on the train tonnage. Special Rule 16 in Time Table 131 (effective Aug. 31 1947) requires one retainer for each 40 or more tons in trains with steam power or diesels lacking dynamic brakes. Trains with dynamic-equipped diesels (and all dynamics working) could take 70 tons per operative brake, and that was also the maximum tonnage allowed west from Summit. At times cuts of empty reefers were left at Summit to be picked up by heavy westward trains that needed improve their ratio of tons per brake.
On diesel-operated trains with working dynamics, retainers were "manipulated" from the engine back; on other trains from the caboose forward, to the required number of cars. On trains handling all empties, the retainers could be applied on alternate cars.
The handles on the retainer valves were turned up for the "on" position and down for "off." (In 1947, two-position retainers were not yet in use.) In addition to the two train brakemen working through from Barstow to San Bernardino, three or four swing brakemen rode all freights from Victorville to San Bernardino, so at Summit there were five or six men available to operate the retainer valves. Fred Carlson's article, "Air brakes for model railroaders," in the November 1994 "Model Railroader," includes a photo of a brakemen using his brake club to reach down from the running board of a car and turn up a retainer valve at Summit. (Fred also went into more detail on how retainers worked on each car and in a train.)
Train sheets show that except when waiting for other trains, most freights stopped for only 15 to 20 minutes, so they got this done expeditiously. In that time the crew also inspected the train and performed a standing set-and-release air test.
With retainers applied, the engineer could apply the brakes to control the train's speed and then release the brakes to recharge the cars' reservoirs. While in release, the retainers would maintain some pressure on the brake shoes to keep the train from gaining speed too quickly. Still, trains descending under retainer control would alternately speed up and slow down as the engineer operated his brake valve. With dynamic brakes the descent would be smoother and speed could be more easily controlled, but the dynamic brakes of the 1940s were relatively primitive compared to later developments.
Running downgrade with some brakes always applied heated the wheels more than normal and could lead to other kinds of trouble, so the trains stopped for wheel cooling and inspections for ten minutes each at Cajon and Devore, and the brakemen rode the tops of the cars looking for problems all the way to San Bernardino (day and night, in all kinds of weather – think what that says about those railroaders).
Trains stopped too at Highland Junction, 1.9 miles out of San Bernardino, to turn down the retainers. I think that covers all the questions you raised, at least as far as 1940s practice on the Santa Fe is concerned, but let me know if there's anything else I can add.