I split out parts of a previous thread posted by Glen Icanberry so that original intent of it being a memorial is respected versus the technical discussion here. Comments are my opinion.

> West of Gish along the old South Track is the unofficial name of Drawbar Flats, at approximately MP 60.1 to MP 60.9. For westward trains, coming down the 3.0 percent grade, the grade reduces briefly to 1.73 percent at MP 60.3. Continuing westward, the ruling grade then increases to 2.36 percent, at about MP 60.4, then at about MP 60.5, the grade abruptly drops off, resuming the 3.0 percent grade. All downhill, westward trains deal with the change in grade while negotiating curve Nos. 86 and 87, and tend to slow down, due to the reduction in grade, heavy braking, and friction or drag in the curves.

> These compensated curves were intended to prevent eastward trains from stalling out when working hard uphill. It has been speculated that because these curves were graded as compensated curves, the curves could have been re-aligned to eliminate, or reduce, the lesser gradient. However, possibly due to the North Track being constructed in 1913, intended as the Eastward Main Track, this work was not done. Increased drag, from the combination of wheel flange friction, against the curved railheads, and the force of gravity on trailing tonnage, tends to slow longer eastward trains, working uphill, in addition to slowing downhill trains, as previously mentioned.

Due to how early they were graded, I'm not sure they were compensated grades, though the later (1967?) SP grade is compensated. A true compensated grade reduces the grade through curves so that the train resistance going uphill theoretically remains fairly constant, though I'm not sure if any consideration is given to a locomotives partial loss of adhesion on curves.

As stated, the increased train resistance through curves work against you going uphill, and can work for you going downhill. But curves in a steady or compensated grade will mean that the train will slow going through them when going downhill.

Based on the formulas of 20# per ton per % of grade for resistance and 0.8# per degree of curve per ton, the grade must be reduced 0.5% for a 12.5 degree curve. Its been a while, but I don't think Cajon Pass has anything that sharp or sharper than a 12 degree curve, so the reduction in grade from 3.0% to 1.73% or 2.36% is not a true compensated curve but more likely due to local geography and the limitations of early surveying.


> In the days before composition brake shoes and diesel electric locomotives with their dynamic braking, the old cast iron brake shoes lost efficiency with increased downhill speed. Conversely the cast iron brake shoes gained efficiency with loss of speed. This meant that locomotive engineers were on the very edge of good train handling, while controlling air brakes, and slack action, utilizing their trains’ momentum, in addition to using or negating the braking effect of the curves, while passing over the disrupted gradient.

Cast iron brake shoes have a friction curve that resembles a tractive effort curve, and they tend to grab harder, but not substantially different until speeds are much slower. Cast iron brake shoes have about a 15% efficiency compared to 30% for a high friction composition brake shoe, but high friction comps have a much flatter friction curve. Car designs were able to change their brake rigging design to only provide half the brake shoe force with comp shoes and obtain the same braking, and locomotives switched from clasp shoe arrangements (one shoe on each side for two per wheel) to a single shoe arrangement. While engines with cast shoes are prone to slide at low speeds, the high friction comp shoes did not provide enough static holding force when stopped resulting in railroads increasing the independent brake pipe pressure (typically from 45 to 72 psi) to hold stopped trains on a grade.

Remember that almost the same amount of horsepower or tractive effort used to climb a hill has to be created by the dynamics and brake shoes going down the hill. There is a limit as to how much braking horsepower each wheel and brake shoe can create before the heat generated can not be dissipated fast enough resulting in a thermal failure of the brake system. There is a known phenomenon known as "brake fade" that as the brake shoes heat up, they can and will lose some of their friction resulting in less braking. So far "brake fade" has eluded efforts to create a theoretical formula to be able to accurately predict its effect and program simulators. The maximum speed during heavy braking has to be reduced, as doubling your speed doubles the amount of heat produced which may exceed the thermal limits of wheels and brake shoes.

On steep grades, there is also a real possibility that the train air brake system does not have sufficient capacity to control the downhill speed while keeping enough braking effort in reserve to stop within a reasonable distance. This can be the simple lack of braking effort (by car design, maintenance or overloading), the engineer pissing away their air (insufficiently charge/recharged brake system) or a brake failure due to exceeding the thermal limits.

> In the old days, the accepted method of downhill train handling was known as short cycle and release. As westward tonnage started down from Summit , even with retainers properly setup, a brake pipe reduction or heavy brake application, “set” of the train brakes was made. The amount of this reduction, or brake application, was calculated by engineers through consideration of the trailing tonnage, but primarily through experience, or seat of the pants “feel.” Sometimes the initial brake application, utilizing the old No. 6 or No. 8 brake valves, was too much or too little, resulting in the train bogging down through Drawbar Flats, or taking the curves too fast.

> The old No. 6 and No. 8 brake valves did not allow rapid recharging of the air reservoirs on the cars, therefore going to full release, and possibly running away downhill, was not the best option. If the train bogged down too much, in the area where the grade was reduced, pulling on the train to maintain speed was sometimes necessary. Under many of the outlined circumstances, the grades, curves, braking and/or pulling, caused slack action, which resulted in yanked out drawbars, (broken coupler shanks), and/or broken knuckles. Thus the unofficial name of Drawbar Flats was bestowed upon this location.

You are correct in that the short cycle method of braking was previously used in conjunction with retainers, but you missed describing how it works. On the older brake valves, an engineer would make a brake pipe reduction to set the brakes, and then "lap" the brake valve. After the brake pipe blew down through the equalizing piston, when in lap, the engineer's brake valve does not increase nor reduce the pressure in the brake pipe. On most trains, there is a normal amount of brake pipe leakage through the various pipe and hose connections, and the gaskets. The long standing rule has been that the maximum brake pipe leakage cannot exceed 5 psi per minute. If a train makes a 12# reduction at 15 mph, and has two pounds per minute brake pipe leakage, within 6 minutes or 1.5 miles the brake pipe reduction would have doubled to 24 psi (almost a full set) hopefully stalling the train. Freight brakes are "direct release" - that is you can gradually apply the brakes, but they must be fully released to allow the brake system to recharge.

Retainers originally had three positions, EX (normal direct exhaust - handle straight down), LP (low pressure or 10#) and HP (high pressure or 20#). Passenger cars and cabooses can have single pressure retainers. This evolved to a four position retainer adding SD (slow direct), and in 1967 the standard changed to a new three position EX-HP-SD. Direct exhaust or "turned down" is the normal position that allows the brakes to release normally. When "turned up" or LP-HP-SD, the brake cylinder exhaust is restricted to take twice as long for the brakes to release. Additionally, in HP or LP, a spring retains the last 20 psi (HP) or 10 psi (LP) of brake cylinder pressure and prevents it from releasing entirely.

Based on the tonnage of the train versus how many operative brakes it has, the train will stop and turn up retainers at the top of a grade starting on the head end. HP is used for loaded cars, and LP (now SD) is used for empties. Depending on the rules, some or all of the retainers may be required. As a train begins to stall due to the brake pipe reduction being increased by brake pipe leakage, curve resistance or lessening grade, the engineer can release the brakes. With retainers turned up, the brakes take twice as long to release, and cars in HP or LP position will retain some braking effort, delaying how fast the train will start to accelerate. After allowing the brakes to recharge, and before the speed increases excessively, the engineer makes another brake pipe application. On those cars with retainers set in HP or LP, the new brake application is added to the brake cylinder pressure retained by the retainer. (A retainer in HP position retaining 20 psi of brake cylinder pressure approximates an 8 pound or minimum brake pipe reduction.) The engineer continues the set and release cycle down the hill.

There is a fine line between using the short cycle method and pissing away your air resulting in a runaway. Unless enough time is allowed for the brake systems on the cars to fully recharge, each subsequent set has to be deeper to obtain the same braking effort. You also have to reserve enough braking to potentially stop and not use all of it to just keep your speed from increasing due to the grade. Run out of air, then you run away. Typically the rules require a train to stop every so many miles for at least 10 minutes to allow the wheels to cool, which also gives a chance to fully recharge the brakes.

Actually #6 and #8 (and some #24) brake valves had both a release position that allowed main reservoir air to bypass the feed valve and flow directly in the brake pipe. The engineer could leave the brake handle in this position on long trains until the brake pipe pressure got close to the feed valve setting in which case the handle was moved to running which used the feed valve to charge the brake pipe and system. If the engineer left the brake handle in release for too long, as the brake pipe would settle back down to the setting of the feed valve, any of the head end cars that had overcharged would set their brakes. The reason most long trains of the era stopped before releasing the air brakes at low speeds is because a full release of the brake system was not always possible and the resulting slack caused issues. The introduction of the ABD control valve and its "accelerated release" feature where a slight increase in brake pipe pressure will cause the ABD to dump emergency reservoir pressure into the brake pipe (accelerating the serial action release) is actually what allowed running releases of long trains at lower speeds. There is a restriction in the older brake systems that depends on the capacity of the feed valve that is much less on 26/30 schedules.

> The improved 24L then 26L brake valves, along with dynamic braking, made the present method of balancing the grade possible, along with the virtual elimination of the need to stop, for the purpose of turning up retainers.

The original 24RL brake did not have pressure maintaining, but later it was a factory option and many railroads either purchased a factory modification or figured out how to do it themselves. 26L, 30 systems and the WABCO/NYAB/KNORR electronic brake valve schedules all now have pressure maintaining, as well as the rare 6BL retrofitted with a NYAB system. This makes a big difference in how an engineer can handle a train down a long grade. With "pressure maintaining", the engineer's brake valve maintains the brake pipe pressure at the specified level. As long as the equalizing reservoir system doesn't have any leakage and the brake pipe leakage is at an acceptable level, it will replace any brake pipe leakage. When an engineer starts down a long hill, if he makes a 12 pound brake pipe reduction to keep his speed steady ("balancing the grade"), then the pressure maintaining system will keep the brake pipe at the needed pressure. The engineer does not have to release the brakes unless they have set too much air, reached a flat spot in the grade or reach the bottom of the grade.

Pressure maintaining, usually in concert with dynamic braking, is what eliminated the need for retainers in most modern situations. Typically on a large train, the engineer will by the seat of his pants make a brake pipe reduction that with full dynamics should keep the train under control on the steepest part of the grade. They will then vary the dynamic braking force to lower levels through the lesser parts of the grade or where curves are holding the train back.


> Locomotive engineers make an initial heavy application of the train brakes, while tipping over the grade at Summit , moving west. Engineers then regulate the downhill speed of their trains, by increasing or decreasing the amount of dynamic braking force. This makes for a smoother, and better controlled, westward descent of Cajon Pass , thus resulting in far fewer undesired emergency brake applications, along old Drawbar Flats.

> Experienced locomotive engineers described allowing the train speed to creep up by just a few miles per hour, approaching Drawbar Flats, then allowing the lesser gradient to drag/slow the train down to about 17 to 20 mph, thereby sliding right across Drawbar Flats without bogging down, or causing severe slack action. This also requires keeping dynamic braking at a sufficient level of retardation, that does not allow the locomotive consist to “run out” ahead of the train, thereby causing slack action.

My guess is that most of the busted knuckles and drawbars were the result of improper train handling. If an engineer has too much air set and won't or can't release it, they might try to pull their train through the potential stall. This change from bunched to stretched and back to bunched slack is where you will get in trouble. An engineer on a CSX coal train actually ended up with a runaway by overheating his air brakes pulling through a sag at speed, unaware a bad mu cable prevented the dynamics from functioning in the rear of the consist.


The use of pressure maintaining (eliminating the need to cycle brake in most instances) and dynamic braking (which assumes a substantial portion of the braking requirements) has eliminated the need for retainers in most instances. Engineers are no longer taught braking theory or how to even use retainers.

The FRA in the last few years required the railroads to change their air brake rules. Too many railroads became reliant on dynamic brakes, and were operating trains incapable of control by air alone. Too many trains were having dynamic brake failures and engineers did not recognize the danger, and in trying to control their trains with air lost control.

The current (as far as I know) Union Pacific rules regarding retainers:

34.5.5 Retaining Valves

Retainers may only be used after consulting with a Manager of Operating Practices for the location involved.

When retaining valves are used:

Retaining valves must be set in the "HP" (High Pressure) position on the entire train.

Do not exceed 15 MPH.

Freight car brake cylinder pressure is not retained until a brake pipe reduction of at least 10-psi has been made and released. Further brake pipe reductions will add to this pressure in the brake cylinder.

When retaining valves are not in use, place them in EX(Exhaust).