Without dynamics and pressure-maintaining locomotive brake valves train crews were required to set retainers in order to safely descend long, steep grades. Sometimes trains also had to make stops to allow the wheels to cool before proceeding.
A retainer is a valve found on the exhaust pipe from a car's brake cylinder. It's normal position (Direct Exhaust) allows the air brake to release normally, but the retainer can also be set to different positions which make the car take longer to release (Slow Direct), or not release completely (High Pressure). Older cars had a "Low Pressure" position too, which kept a lighter brake application than "High Pressure".
Pressure-maintaining is a feature of the locomotive's automatic brake valve which adds just enough air to the train's brake pipe to compensate for leaks, but not enough to trigger a release.
To understand the rationale behind these features one must learn a bit more about how the air brake system works. Once the system is charged one reduces the brake pipe pressure to set the brakes, and any futher leakage will result in a heavier brake application. When descending a long, even grade without a pressure-maintaining brake valve this leakage will eventually result in the brake application becoming heavy enough that the locomotive(s) cannot pull the train downhill anymore (even in full throttle), so the Engineer must release the brake in order to continue.
A car's air brake (except for some passenger equipment) can only be released completely, the Engineer cannot back off a bit to a lighter application. It only takes a 1 or 2 PSI rise in brake pipe pressure to trigger a release. The brakes will then release fairly quickly, long before the system has fully recharged. Now the Engineer must make another application to control the train, but will have to make a larger reduction in brake pipe pressure in order to achieve the same braking effort, as the system is not fully recharged yet. Doing this is called Cycle Braking, and doing it too many times will result in not enough air left to control the train. The technical term for this is "Peeing away the Air", and it has caused many runaways over the years.
This is where the retainers come in. Setting them to "Slow Direct" allows the train to recharge for a longer time before the brakes release, and setting them to "High Pressure" prevents them from releasing completely. Setting enough retainers to either position gives the Engineer more control.
Though they are rarely used anymore every car is still required to have a retainer. Today they are normally found at a lower level somewhere around the side of the car, but historically they were placed high up on the end of the car, right next to the stemwinder handbrake. This placement allowed both handbrake and retainer to be operated while the train was moving, with the Brakemen walking along the tops of cars.