As I understand it, If the locomotive has sufficient braking ability and the cars have sufficient reserve air to keep the brakes from dynamiting, they may not hook up the air to the cars at all.

Yes, that's correct. When flat switching cuts of cars, the loco brakes are generally sufficient.

Even in a long train, the loco brakes can be used to alter the effects of train brakes to fine degree, adding the ability to "fine tune" the break set through curves, etc.

I thought that without air pressure, the brakes on cars was by default in the closed position. Is that not the case? They don't need to pressurize the car system even just to move it?

You just asked the complicated version of the question.  This is the two-cents version of the answer:

The mechanical brake system on each car operates via mechanical linkage from the brake cylinder on each car.  This cylinder activates by air pressure from the service reservoir on each car, which was initially charged from the trainline (brake pipe).  This air application in turn is controlled by the triple valve on each car, which does one of three things, depending on the pressure in the brake pipe.  In all three cases, the triple valve is comparing the brake pipe pressure to the reservoir pressure.

If the brake pipe pressure is higher than the reservoir pressure the triple valve moves to the RELEASE position. If the brake pipe pressure is lower than the reservoir pressure the triple valve moves to the APPLY position. If the brake pipe pressure is equal to the reservoir pressure the triple valve moves to a neutral or LAP postion

If there is nothing in the reservoir (i.e. the air has been bled off), then there are no brakes; no air pressure to apply the brakes.

Trains are very typically assembled in the yard without any brakes used except the locomotive brakes.  Once the train is assembled and all hoses attached and valves opened, then the system is charged with air.  This can take 10, 20, or even 30 minutes on a long train, to fill the train line and all the reservoirs on all the cars.  Some yards have shop air compressor air piped to yard tracks to pump up train air, so the road locomotive doesn't have to.

Thans Don, for that great explanation.  Makes perfect sense to me now.

If there is nothing in the reservoir (i.e. the air has been bled off), then there are no brakes; no air pressure to apply the brakes.

And that's what causes a runaway - when frequent braking through mountainous territory bleeds off all the air pressure in the system and there's nothing there to apply the brakes anymore.

Air pressure (in the reservoir) is required to apply the brakes. The control valve detects a decrease in the pressure in the line as a signal to apply the brakes so the system fails safe - if the line is severed, the pressure rapidly drops and the brakes are applied (automatic emergency application).

Each freight car actually has two air reservoir tanks. The round tank you see on prototype and model is actually split in two. Thats why there is an air pipe coming out of each end. One is for regular use of the automatic brakes, and the other is the emergency tank, used for when the train needs to go into an emergency application.

For modeling the correct "air-time" in modeling operations, give your switchmen time to "bleed the air" off a string of cars set out by the road feight before they start kicking them. Give the conductors time to walk to the rear of the cars on his pick up and to lace the hoses and do safety inspection of the cars. Then time to charge up the air, ask the engineer to set the air, and then 'walk the set' up to the head end.

I like to imagine that my HO scale conductors and switchman are not into walking great lenghts on rock, just as thier real brothers are. So all trains must be stopped when throwing a switch, and must be stopped within one foot (real foot..12 inches) of the turnout to be thrown. If you want to add scale work time to a layout, stop the practice of walking ahead of your train and lining the route.

Thomas

I was given seminar on maintenance of the Carrier-Transicold refrigeration units used in railroad service on the BNSF, UP, and Tropicana Juice train cars.  Part of the focus in that course was on "blue flag" safety laws and individual railroad requirements.  The instructor for that part of the course was the car superintendant of the BNSF railroad.  He mentioned that BNSF requires the locomotive to stop 50 feet short of a switch (if I remember correctly) before the switchman was allowed to get down and throw the switch.  The reason was that a locomotive parked to close to the switch could put a strain on the switch mechanism that could cause the linkage to "kick back."  If the handle for the switch was of the "ball end" type, that ball kicking back could be lethal.

Here is what happens when switching with only engine brakes and something goes wrong. Unfortunately it is in Finnish, but at least there are some pictures. www.seisake.net/onnettomuus.php

To make the story short. Train arrived from right, red and blue line. The yellow track was under repair so the train needed to be pushed first left, green line, and then pulled back right. When pushing left, the up slope 1% changes to 0.95% down slope. The engine started braking but it could not stop the train doing 15 mph in slope, even with the sand. They contacted dispatcher, but as the train was long and past some switches, they could not divert it towards another up slope. It ended up through the terminal depot wall at about 10 mph. Lesson learned, make doors at the depot big enough for cars...

ps. no people was hurt, physically at least.

Way back late '50's early '60's when I was on espee in West Oakland it went like this: as soon as the road engine cut off the brakes dynamited when the hoses separated, an initial inspection was made and defects carded. As each car was inspected the air was bled off the tank. The switching was all done with engine brakes. When a train was made up the hoses were connected andthe journal boxes topped up with packing waste and oil. Upon completion the yard air connectrd to charge the trainline. The brakes were tested and the cars were checked for application/stroke and those needing adjustment were noted . The brakes were released and after the adjustments were made there was a second static trst before turning the train over to the road engine. The road crew made a final air brake test application and release just before departure. John.