Okay, I know that this has been discussed in other threads, but I’m a bit confused, so bear with me. Over in Germany, rolling stock is equipped with so called “selbsttaetige Druckluftbremsen”, meaning that the brakes are automatic, that is, the brakes are always applied when there is NO pressure on the brake pipe. They can be released manually, car by car, in the yard or shop, but otherwise you need pressure in the air pipe to release them. If a train breaks apart, the breaks automatically apply. Reading some of the information in other threads, it seems to me as if North American rolling stock has brakes which can be APPLIED via pressure on the brake pipe (or manually, of course). Is this correct? If so, why not adopt the German idea? It seems safer to me.
Al Krug explains it very well. Check out his site at
Lee,
Your understanding of North American practice is incorrect. Train brakes are applied by reducing brake pipe pressure. Rapid loss of pressure causes emergency brake application, that is, a harder and faster set of the brakes than a service application. Individaul cars may be bled off for switching.
Locomotive brakes can be applied with the train brakes, released when train brakes are applied, or applied “independently” of train brakes. The independent application is, I believe, a straight air application direct from the main reservoir. This may be the source of your confusion.
Mac
Al Krug’s website does a wonderful job of explaining the concept for a non-tech guy like me. Thanks for the tip! So maybe one of you could explain to me, since the FRA is considering mandating electronic brake control on each car, why not just convert the brake system totally to hydraulic with disc brakes like on an automobile? With it being a closed system, you wouldn’t run the risk of running out of air on a steep downhill grade, and you could do a controlled release. Is it not possible to get the same amount of braking power out of a hydraulic system as out of a pneumatic one?
You would need a tremendous amount of fluid, and sealing couplers at each car end. Inspite of this there would be lots of leaks and pollution. The hoses between cars break with time which wwould also cause ground contamination, also how would you make the system safe?. If a hose parts how would you apply the brakes? I can imagine some type of accumulator on each car, but.
Actually, I was thinking of a self-contained brake system on each car, regulated electronically via a bus line from the lead loco. That way, you only have an electronic connection between cars: no messy hydraulic jacks (I know what they’re like, because I used to work Summers on a farm, and a lot of implements had hydraulic jack couplings; that fluid is nasty, dangerous stuff). I’m sure you could configure some kind of emergency fail-safe to stop the cars if they were somehow uncoupled from the rest of the train.
If you go to ECP braking as the FRA is considering, you’ll greatly reduce that problem of running out of air if you do a series of running releases. Here’s why. Right now, the air train line is providing both the control signal to operate the brakes and the power to apply them. If you relieve the system of the control signal responsibiltiy, you no longer have to worry about the rate at which you charge the train line, so considerably more rapid trainline recovery would be possible. Now, all the air must feed through the feed valve on the control stand of the lead locomotive. Theoretically, the main reservoirs on all the locomotives could recharge the train line more quickly.
If you do away with the trainline to supply the power for the brake applications, you’d have to replace it with something. It is hard for me to envision an alternative to stored compressed air that could deliver the high peak power need to give rapid, powerful braking response.
Maybe I’m missing something, but…
Why mess with success? I am completely in agreement with the concept of electrically signalled braking, as is being proposed by the FRA. Much better train handling, much quicker stopping – and the real beauty of it is that while it is better than the present system, it’s compatible with it.
But the present air brake system works. Reliably.
Is there a description on the Internet somewhere of how this would work?
If I understand it correctly, brakes would be applied via electronic signal while maintaining full pipe pressure. As such, the brakes could be adjusted (more brakes, less brakes) at any time during the braking operation. And yet, loss of pipe pressure would still cause emergency application. Is that pretty much it?
That’s pretty much it - as I understand it. I don’t even know that in the long run you’d even need the 0 psi brake pipe = emergency. Maybe a lack of response to polling from the control source would effect some gradutated braking. There are still too may wrecks caused by UDEs with the “wrong” combination of train makeup and terrain.
“Reliably” is a real push! There are still all sorts of problems with air brakes. Brakes stick, brakes can leak on - or off, there are undesired emergency applications (UDEs) that can be caused by the application pressure wave boucing off the last angle cock and making pressure nodes and antinodes in the brake pipe. There are issues with frost in the valves causing them to stick when you charge a train in a warm humid climate and then operate into the cold. And, it takes a year and a day to get a long train’s brake pipe pumped up through that feed valve on the head end.
Most RRs would prefer to get trains over the road without the engineer every having to use the automatic. I think most engineers would prefer to leave it alone, too.
In this computer age, is anyone, anywhere still using pneumatic contol signals to operate machinery?
Yes!
Although most to the pneumatic systems are “legacy” systems, there is no reason to change it if it works!
“Modern” control systems use a 2-wire current loop (4-20mA signals) but the most up to date are “Data Bus” controls, using twisted pair for signaling. (Twisted Pair = telephone (cat3) or Computer Network (Cat 5 etc)
I did an Instructors summer internship at Intel in Rio Rancho (albuquerque) a few years ago, and imagine my surprise to find that the interlocks on a $10M dollar Ion Implanter was actually using low-pressure Pneumatics to control a safety interlock!
The most current and modern instrumentation and control systems are the computerized ones, but there is a lot of stainless steel tubing instrumentation used in modern process control applications!
Same reason we have DPU’s - longer trains. They’re talking 12,000’ long trains with ECP. And as we all know, bean counters usually win these interfirm proposals.
I’m gonna warm this thread up again, 'cause I’ve been thinking about something, and I just can’t figure it out. I’ve ridden on commuter trains frequently. Now, I am fully aware that the length of a typical commuter train is miniscule compared to that of a North American freight. But in a metropolitan area, with the train accelerating to about 60 mph, but making stops on the average every 60 seconds, how could the brake pipe recharge fast enough after a station stop to effectively stop the train in time for the next station? Does each car have its own, independent air pressure system? And is this seperate from the pneumatics used to close the doors?
The brake pipe recharges from a main reservoir so that the air compressor can keep up with the demand, running while the train is stopped as well as moving. Also, it’s likely the train has blended braking where the dyanmic brake does a good portion of the braking. And finally, if it’s an MU, each car would have it’s one air supply system, although the brake pipe is only fed through the head end but the main reserviors on each car would be connected.
The brake pipe recharges from a main reservoir so that the air compressor can keep up with the demand, running while the train is stopped as well as moving. Also, it’s likely the train has blended braking where the dyanmic brake does a good portion of the braking. And finally, if it’s an MU, each car would have it’s one air supply system, although the brake pipe is only fed through the head end but the main reserviors on each car would be connected.