Dan mentioned that some railroads combined the straight air/automatic air systems. The Durango & Silverton did this in 1982 under the skillful supervision of Dan Nichols, the car foreman at the time.
In the D&S system, each locomotive is fitted with an additional train brake valve and an additional train line feed valve (pressure reducing valve). All the coaches and miscellaeous cars used in daily passenger service are fitted with a second air line that runs the length of each car and is connected to the adjoining car with a hose and glad hand similar to the main train line. A branch Tee in the additional line under the car connects to the triple valve exaust line, which extends up the rear of the car to the retainer valve. There is a cutout valve below the retainer valve to prevent exhausted air from the brake cylinder from exiting via the retainer valve.
Under normal conditions, the train is set up for combined straight air/automatic air operation. This requires that the straight air line on each car be connected, as well as the main (automatic) air train line, and that the retainer cutout valve on each car is closed.
When the locomotive is coupled to the train, both the straight air and automatic air lines are connected up. With the H6 automatic air valve in the cab placed in the "running" position, air from the main air reservoirs at 130 psi passes through the 90 psi train line feed valve and out via the train line to the auxiliary reservoir under each car until all the auxiliary reservoirs are charged to 90 psi. Meantime, the straight air valve in the cab, which is simply a three-position rotary valve, is also in the "running" position. In this position, no air passes into the straight air train line.
Underway, to apply the brakes with the straight air system, the engineer moves the straight air valve from the "running" position to the "service" position. Now 130 psi air from the main reservoirs on the locomotive passes through the straight air feed (reducing) valve where it is reduced to 55 psi and then through the straight air valve into the straight air line. On each car, the air flows into the branch Tee which leads into the exhaust port of the triple valve and straight into the brake cylinder on the car, thus applying the brakes. To hold the brakes at a given setting, the engineer moves the straight air valve to the "lap" position, which shuts off the incoming 55 psi air and traps the existing air charge in the straight air line and all the car brake cylinders. To release the brakes, the engineer moves the straight air valve back to the "running" position, which blocks off the 55 psi supply air and opens the exhaust port, dumping the air in the straight air line and all the car brake cylinders to an exhuast line that exits below the cab.
The benefits of the dual system are manifold. One of the main benefits is that as the train is brought down the hill with the straight air system, the auxiliary reservoirs under each coach remain fully charged at 90 psi, ready and waiting to be used at any time. There is no danger of "pissing away" the auxiliary reservoirs through excessive cycling of the brakes. Not to say that the engineer can't piss away his air by mis-handling the straight air brakes, because he can, though it would be highly unlikely, and he would know in advance if he were using too much air simply by keeping a close eye on the main air reservoir pressure gauge. In the event of an emergency, the engineer can "big hole" the automatic air valve (go into emergency), and the full 90 psi stored in each auxiliary reservoir would immediately flow into the brake cylinders. The two systems can be used together, for example when approaching a water stop. With the brakes partially set up with the straight air system, and the locomotive approaching the spot, the automatic air can be applied to quickly increase the braking pressure on each coach, stopping the train in exactly the right spot. Note that in this case, the automatic air system is applied on top of the straight air system. If, instead, an automatic brake application were made and held first, I'm not sure what the effect of making a straight air application on top of the automatic application, as the triple valves are no longer in their normal "running" position with the brake cylinders ported to the exhaust port. I never tried it.
Another benefit of the straight air system is that it gives the engineer a "graduated release" feature, which is not possible with the automatic air brakes. In other words, with the automatic system, if the engineer sees that he has applied a little too much air and the train is slowing more than he wants it to, he has to release the brakes completely, then come back with another smaller application to get the desired amount of braking. With the straight air system, if the engineer feels that he is slowing down too much, he simply releases some of the air in the brake system by briefly putting the straight air valve in the "running" position, then back to "lap" to hold the brakes at the new reduced braking pressure. The overall effect of this is that the engineer has a much better "seat-of-the-pants" feel and control of train speed when coming down long down grades. Also, it uses considerably less air than the automatic air system, because it is not necessary to release all the air from the brake system each time the brakes are eased off or released. When easing off the brakes, the engineer releases just enough air to keep the train speed as desired, then puts the straight air valve back in the "lap" position, trapping the remaining air in the brake system. This holds the brake pistons partially extended so that all the lost motion (slop) in the brake linkage is out and the brake shoes are in contact with or in very close proximity to the wheels at all times. During the next brake application, very little air is wasted taking up the lost motion in all the brake linkage under each car. With the automatic air system only, except for the rear two or three coaches, which may have retaiers set, each time the brakes are released, all the air from the coaches without retainers set is exhausted, so that on the next brake application, all the lost motion must again be taken out, thus wasting precious auxiliary reservoir air.
Yet another advantage of the straight air system is that the engineer and fireman are not forced to listen to the incessant blast of air exiting from the brake pipe through an orifie in the bottom of the automatic brake valve, inches from the lap of the engineer each time the brakes are released. Three hours of listening to that on a hot afternoon gets really old. With the straight air system, air is released through a relatively large pipe that exits below the cab. It's loud enough that the engineer can hear it, which helps him gauge how much air he is releasing, but it's not at all obnoxious like the automatic air.
One of the disadvantages of the straight air system, as pointed out in other posts, is that if the straight air train line breaks, the straight air system is rendered inoperable. With the combined system, if the straight air line were to break, the engineer would notice right away that a brake application with the straight air valve is not causing the brakes to be applied. Instead of jumping out the window, all he has to do is apply the automatic brakes and bring the train to a normal (or emergency, if required) stop. The damage to the straight air line can then be repaired or the system cut out completely if necessary.
I expect the straight air component of this combined system would be problematic on long trains of 30 cars or more due to the time lag between the brakes on the rear of the train and the head end setting up and releasing, resulting in excessive slack action. However. on the D&S most trains are 15 cars or less, so that is not a problem.
I've often wondered why the combined system hasn't been applied to the C&TS equipment. It's really relatively inexpensive in that the only specialty equipment required is the straight air reducing valve and the straight air brake valve, both installed in the locomotive. The rest of the entire line is simply pipe and pipe fittings, hoses and glad hands. I believe the D&S uses a 6ET brake feed valve, tweeked up to 55 psi from the normal 45 psi for their straight air feed valve. The straight air brake valve is an off-the-shelf three-position rotary valve common to pneumatic and hydraulic systems. The additional safety of having a backup air brake system on a tourist operation with steep and prolonged grades would seem to justify the added cost of the system.
