What are these cars for and how do they work?
Think of them as just the automatic air brake portion of a DP unit (they link to the lead unit using the same computer and radio systems, and show up on the DP screen as a remote consist). They are there to provide and additional source of air to the train and allow for longer trains to be operated in cold weather.
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So the DPU units are not supplying air to the train line?
I expect SD70dude to have a full account of the Canadian midtrain air cars, which I think is the OP subject.
The great advantage of ECP braking over one-pipe Westinghouse is the speed at which the individual car valves actuate. While an emergency activation is triggered by a pressure wave in the air line (explaining why the emergency performance of ECP is only about 3% greater than conventional) service braking application is modulated by fall in trainline pressure, which propagates much more slowly if the only exhaust is in the cab.
This was addressed in the FREDs, which have a separate air valve that can be set to work at the other end of the consist, venting air from 'both ends'. This in theory allows quicker application and 'set' to a given level of braking. (Note that for reasons too complex to give here, you can't release the brakes with the same graduated performance you can apply them, like passenger brakes, which is another great advantage of ECP...)
Now, there can be problems with low brake pressure, or low recharge rate, that affect train handling, especially in severe Canadian weather. The Canadians developed a system with air compression and vent valves in a slip-in shipping container shell, which can both assist in pumping up the trainline if cold or leaky, as well as spill air in the middle of the consist as well as the ends.
I was unaware that DPUs did not help pump up the trainline.
DP units pump air into the train line and exhaust it during brake applications.
Current FREDs can only dump air. They don't make graduated applications in tune to the lead unit.
Freight car control valves aren't designed for graduated release. I'd guess the reason freight cars aren't equipped with graduated release control valves is cost. Both to outfit and to maintain the equipment.
We have instructions that when the temperature is 10 F and less, not to use the distributed power train check feature between crew change points. (Train check cuts out the automatic brake valve on the remote consist leader. When the air brake is released and the remote(s) cut the valve back in, it shows the train line is intact.) It's because in very low temperatures the automatic brake valve has sometimes frozen up.
I can just imagine what it's like where winter temperatures that cold and colder happen way more often than Iowa.
Jeff
jeffhergert Freight car control valves aren't designed for graduated release. I'd guess the reason freight cars aren't equipped with graduated release control valves is cost. Both to outfit and to maintain the equipment.
My understanding was that graduated release created problems on long trains, hence its use being confined to passenger cars.
Erik_Mag jeffhergert Freight car control valves aren't designed for graduated release. I'd guess the reason freight cars aren't equipped with graduated release control valves is cost. Both to outfit and to maintain the equipment. My understanding was that graduated release created problems on long trains, hence its use being confined to passenger cars.
And is also the reason CSX has a maximum car limit on Auto Train. I am not sure but I think the limit is either 44 or 48. My casual observation is that works out to 12 passenger occupied cars and the balance being auto racks.
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Makes sense. That was one of the draw backs of Westinghouse's original straight air brake system. Long trains had trouble with brake operation towards the rear of long trains.
The Auto Train's length should also be limited by Amtrak's HEP system, where only one unit can supply power to the passenger cars.
Using EOTs to propagate service brake applications has been tried. This feature was called Brake Assist. It had problems with the EOT's valve sticking open, which would cause a heavier and heavier brake application toward the tail end of the train and eventually drag you to a stop, even if the tail end 'anchor' didn't dig in hard enough to cause a train separation first.
CN's Distributed Braking Cars (more commonly referred to as 'AirCars' on the property) contain a small diesel engine, generator and of course an air compressor. From what I could see they use the same air brake parts and controls as a modern locomotive, the breaker panel and air brake rack look very similar to what you would find under the cab or on the rear cab wall of a newer GE. They react to the lead unit's automatic air brake commands exactly the same as a DP remote locomotive, link to the lead unit using the same DP system, and show up on your DP screen as a remote consist. They differ from DP locomotives in that you don't have to go back to the AirCar to set it up as a remote, when not linked to a lead unit they are always ready to be linked to any unit, just set up DP or add an additional remote consist on the lead unit, type in the AirCar's number, and you're ready to link it. Don't forget to unlink the AirCar when it is removed from your train, or it won't be able to link to another unit later on. Once linked you do the same brake pipe test or train check you would do with a remote locomotive, and you're ready to go.
The boxcars and containers have the same equipment inside, the boxcars just have a lot more empty space. They also have the same stash of spare parts and basic tools you'd find on a locomotive, along with a spare EOT in case the AirCar breaks down.
When there are working the AirCars work just as well as a DP remote locomotive for the purposes of supplying air to the train and speeding up applications and releases. In years past they seem to have had a higher failure rate than locomotives but I think this is improving over time, and they are once again out in full force this winter (they usually go into storage during the summer).
Because they do not produce tractive effort and also do not have an independent brake it is even more important to consider the state of your slack (stretched out vs bunched) if you have an AirCar at or near your tail end. With a remote locomotive you can keep the independent applied or start pushing to keep the tail end from rolling away backward when the brakes release, with an AirCar you can't and it will also cause the tail end to release faster than the middle of the train.
When an AirCar isn't linked to a lead unit it reverts to a sleep or dead haul mode, and its air brake control equipment doesn't react to changes in brake pipe pressure like a remote locomotive would. They still have their own car control valve and act just like any other freight car when not linked, so they can deadhead anywhere in a freight train without issues.
My understanding is that the pressure-maintaining feature is a major reason why freight brakes can't have graduated release.
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