In my opinion - several issues lead to the lack of the use of retainers in modern day railroading: dynamic brakes, pressure maintaining, the time required to use them, the abandonment of severe grade territories and the lack of knowledge.

Early loco brake systems (A-1, 6, 8, 14 and early 24) did not have "pressure maintaining." When the engineer made a brake application, the brake pipe leakage would continue to increase the brake pipe reduction. If you needed a 10 pound reduction to hold your train speed on a grade, with a 2 pound per minute leakage your reduction would be 20 pounds after 5 minutes. If your brake pipe leakage was the maximum allowed of 5 pounds per minute, your reduction would increase to 20 pounds in two minutes. This means that the engineer has to release the brakes to keep the train from stalling. Now the issue becomes that enough time has to be allowed to rechage the auxiliary reservoir on each car in release before having to reapply the brakes to keep from exceeding the maximum safe speed. If you continue to cycle the brakes without recharging the auxiliary reservoirs sufficiently, you end up "pissing away your air" and have a runaway because the remaining air in the cars to apply the brakes is insufficient to contol your speed or stop.

"Turning up" retainers does a couple things. First - it restricts the exhaust of the brake cylinders so it takes twice as long to release. Second - except for slow direct, all the other positions "retain" a fixed amount of air (10, 15, 20 pounds depending on the design and handle position) in the brake cylinder of that car. The delay in releasing the brakes and the minimum air retained in the brake cylinders allows additional time for the brakes to be recharged. The engineer must still apply the brakes before the speed exceeds the limit, and release them before stalling, but the delayed release and minimum braking effort retained allows time for the brake system to recharge.

It takes time to stop a train at the top of a grade and for the trainmen to turn up the retainers. On long grades, additional stops of 10 minutes have to be made to allow the wheels to cool. At the bottom of the grade, a stop has to be made to turn the retainers "down" to the direct release or normal position.

Pressure maintaining has been standard since mid 24RL production, and retrofitted to most 24L and some 6 systems, and standard on the 26L and newer. A pressure maintaining engineer's brake valve has a pressure regulating system that acts to maintain the brake pipe pressure against normal leakage. As a result, an engineer can make a 10 pound reduction at the top of the grade, and if properly working, the brakes will remain at that level until released are applied harder by the engineer. (Most passenger train equipment can partially or gradually release their brakes in steps if desired. Long passenger trains (ie Auto Train) and all freight trains can only release their air brakes completely.)

Dynamic brakes assume much of the braking effort previously done by the train's air brakes. Dynamic brakes do not reverse the traction motors. The traction motor connections are changed to allow them to convert the rotational force of the wheels into electrical energy by becoming generators. The electrical power created is dissapated as heat through the dynamic brake grids. Simply the forces of momentum and gravity are converted to heat, just like plugging a heater into a generator. The same amount of horsepower needed to get a train up a grade has to be disapated to come down the same grade.

Except in a few cases, the use of dynamic brakes and pressure maintaining has eliminated the need for retainers. Unfortuantely dynamic brakes are not a fail safe system, and if a train is assigned or has a failure reulting in insufficient dynamic braking per trailing ton, most railroads require the train to stop until more dynamic braking is obtained or the train doubles down the hill. Very few modern crews are trained or qualified to take a train down a grade on retainers any more.