>Huh? It has much to do with the brake valve. With a non-maintaining valve you will continue to lose air and have an increased application, necessitating a running release and the need for the retainers to hold braking effort while recharging.

All brake valves with (brake pipe) pressure maintaining can do is prevent brake increases that are caused by brake pipe leakage (until the leakage rate exceeds the capacity of the maintaining feature). This goes a long way toward simplifying brake use, but what no brake valve can do is compensate for speed reductions and increases arising from grade changes and other track-related conditions. This means that when a grade levels out for a short stretch, you may still have to make that running release in order to avoid bogging down and/or coming to a halt. In other words, the characteristics of the grade dictate the use of retainers; the type of brake valve can only reduce relying on them.

As I said, those UP locomotives used on the Vasona branch have 26-L, which the last time I checked has pressure maintaining, yet the UP's SSI say to use retainers.

>Leaving dynamics out of the equation, pressure maintaining would have done away with many a territory requiring retainers.

Yes. However, there was, or would be, a provision in the air brake rules which would specify using them whenever the maintaining feature failed.

I'm surprised no one has mentioned the need to stop for wheel cooling when using retainers, although many of the grades which used to require them aren't long enough to have to do this.

The basic concepts and apparatus for (brake pipe) pressure maintaining were invented 100 years ago but weren't put on the market in a big way until after WWII. Bridging and feed valve braking were cheating, and feed valve braking is specifically prohibited in this day and age.