I know that there is a difference between freight braking and passenger braking, but not sure what. Can someone please explain? Thank you!
Freight equipment does not have the ‘graduated release’ feature which allows the brakes to be partially released.
In the locomotive, a 26L brake valve is necessary to use ‘passenger’ equipped automatic brakes. This feature has 3 settings: “freight”, “passenger (graduated release)”, and “out”, whereas a standard 26L has only “in” and “out”. Other brake valves (24RL, 6BL, etc) do not have a passenger setting.
So if passengers cars are being pulled by a locomotive that only has freight brakes, will the passenger brakes still work (but I’m assuming that the brakes will only be full on or full off then, correct?)
Also, I’ve noticed that Amtrak cars have disc brakes instead of the shoes going to the RR wheels - is this just for greater braking ability or is there another reason?
We’re running all old equipment - some dating to the forties. Even though it’s passenger equipment, all of our locomotives are freight.
The brakes work just fine, we just don’t have the graduated release. It might be handing with the hilly profile we run on, but we don’t have it available.
The brakes aren’t just “full on/full off.” They can be applied in steps. They just have to be completely released - nothing partial about that.
It helps with braking ability, and also with braking heating, and the resulting brake fade.
Without digging all my books out the most important difference is the “braking ratio” which is the ratio between maximum brake shoe force applied to an axle and the weight carried on the axle. The braking ratio must not exceed the adhesion ratio between the wheel and the rail. The adhesion ratio is generally about 25% on dry rail and 20% on wet rail, so 20% is the upper limit. The designer may use a slightly lower ratio for safety’s sake. As the ratio goes lower, holding all else constant, stopping distance once the brakes are applied increases in porportion to the reduction in braking ratio.
Since the axle loading of locomotives and passenger cars varies only slightly in service their braking ratio can be 18-20% which gives a corresponding stopping distance from various initial speeds.
Freight car brakes are designed to the same ratio for empty cars. A 60,000 pound tare weight car with a braking ratio of 18% becomes a 263,000 pound load with a braking ratio of 4.1%. That is why freight trains take so long to stop. Most railroads show “tons per operative brake” on their consist and require that the engineer be told what it is. The empty is 30 TPOB while the load is 121. Engineers are fully aware of how trains with different TPOB ratios should handle but most pay close attention to how a new train responds to the first two or three applications they make to be sure that that they get is what they expect.
Mac
Great explanation!
I’ll just add that when designing braking systems for passenger cars, the designer has to keep in mind what the braking HP requirements are since they will determine how much heat has to get dissipated how fast. A full service stop from 79 mph will really heat the wheel tread up and leave the wheels prone to thermal cracking. Discs are better able
I’ve been told that tread braking also keeps the wheels cleaner for better ‘shunting’ of the track circuits for the signals to properly display occupancy and actuate at grade crossings, etc. Not an issue with Amtrak, but with some commuter and light rail lines, if they had only disc brakes, without the wheel cleaning of the tread brakes the contact quality could become problematic. This was allegedly a factor in a grade crossing collision with a RDG or SEPTA RDC about 20 years ago in the Southhampton, Bucks Co., PA area at a major highway.
More importantly, passenger brake systems are set for a higher train line pressure - 110 PSI vs. 90 PSI for freights, if I remember correctly.
Back in the day, some passenger trains were also set up for electrically-actuated braking, but maintenance issues and rotating non-equipped cars in and out of the consists caused that to be discarded. I believe that the Auto-Train operation still has that, though, or at least did until Amtrak took it over, because of its very long length and the weight of the auto-rack cars at the rear.
See also Al Krug’s “Air Brakes” essay at - http://www.alkrug.vcn.com/rrfacts/brakes.htm - especially the sections on “Load/ Empty Sensors” and “Air Pressure Variations” about 2/3 of the way down.
- Paul North.
And similarly, signal spacing is based on the worst performing train, which means that a passenger train is not going to do any better on the signal aspect progression than a freight train.
RWM
The METRA train in Chicago have standard two-shoe tread brakes per wheel. On an hourly-service trip, the train will make an average of 20 stops each direction, with many of those stops coming from 70mph speeds. The locomotives are equipped with “blended braking”, which utilizes a combination of dynamic/air brakes on the locomotive.
Having the “graduated release” feature is almost mandatory for a suburban train due to many stations have very close tolerances for spotting the train for loading/unloading (sometimes as little as 10’ of ‘wiggle-room’).
One of the intersting Factoids mentioned in TRAINS magazine’s recent article about the Ringling Bros, Barnum 7 Bailey Circus trains was that all of their rolling stock is set up with freight car brake systems. I guess because these are very heavy mixed trains with all the livestock and vehicle cars (although iInote that the AUTOTRAIN is about as long and heavy and uses passenger brakes). The circus trains also rely largely on freight locomotives for power…
Right. It would not do much good to have passenger-type brakes on the train cars if one did not have a automatic brake valve with the ‘passenger’ feature on the locomotive.
While they didn’t have a “Passenger Position” as the 26L brake valve may, the old 6BL and 24RL worked just fine at graduated release. The engineer simply moved the brake valve handle from lap position to running, watched the equalizing reservoir gauge hand for the desired increase, and then moved handle back to lap position. Most railroads’ rules prohibited an attempt to graduate off an application of ten pounds or less as some cars might release entirely and some not, resulting in spilled coffee and passengers complaing bitterly.
Too many of my recent Metra and Amtrak rides have been with engineers who use the “Stand 'em up when you stop” and “Sit 'em down when you start” method of braking instead of graduated release.
I don’t want to pretend to be an expert on air brakes - I have just enough knowledge to be dangerous. But my understanding is that the graduated brake feature can’t be used on trains which exceed a certain length (I’m not sure what it is) even if all of the cars are equipped for it. The reason is that, with a long train, the “graduated” release will not be propogated evenly throughout the train - the front of the train may make a partial realease while the rear of the train makes less of a release or no release at all, which makes for unfavorable train handling. I seem to recall seeing somewhere that the Amtrak auto-train doesn’t use graduated release for this reason - it uses the freight brake setting. Electronically actuated brakes, if and when they ever come to pass, will eliminate this problem.
Are you sure the method you described should be described as “graduated release”?
My understanding is that with the older brake valves (like the ones mentioned), when you moved the valve to “running” you were sending brake pipe air back in to the trainline, and by then moving the handle back to “lap” you merely stopped the flow of air. The cars that received enough air to trigger a release did so, and the cars that did not receive enough air simply stayed set. I cannot imaging how one could be quick enough with a brake valve to use your method effectively on a passenger train. I know that I used that method quite a bit on freight trains, but that was back in the days of AB equipment on the freight cars. And would not an ABD brake trigger a release too quickly for such a method to be used?
“Stand 'em up when you stop” is sometimes necessary to stay on schedule. “Sit 'em down when you start” is from advancing the throttle too quickly relative to the amound of braking force that is being used to hold the train in place; but it is also sometimes necessary in order to remain on schedule.
I’m no expert on air brake systems, but I run equipment at a railway museum that has the older brake valves, so I’m familiar with how they work. Older brake valves l have separate “running”, “lap”, and “service” positions (there are some other positions as well, but they aren’t relevant to this discussion). The way you get a brake application is to place the brake handle in the “service” position until the desired brake pipe reduction is made, and then move the handle back to the “lap” position, which more or less holds everything the way it is. To get a partial "releas
One thing to remember except for the emergency position (and maybe the quick release, the position to the left of the running position on the older valves. It’s been a while since I used them and the term I used may not be correct) the automatic brake valve works only on the equalizing reservoir. The brake pipe follows the equalizing reservoir. This was because on a long train if the handle worked directly on the brake pipe, the engineer would have to watch the brake pipe gauge a long time to get the desired reduction.
When you move the automatic handle into the service zone, you’re reducing the air in that reservoir, not directly to the train line. On short trains, you may see an almost instant change on both gauges (eq resv and the brake pipe). On a long train the eq resv will drop first in conjunction to the handle position, then the brake pipe will drop to catch up. The same with a release. Move the handle to the release/running position the eq resv will go right back to 90psi (or whatever the setting is) quickly, the brake pipe will take a while longer.
One trick in freight service if your equalizing reservoir has a leak is to put the handle into the passenger position if so equipped. In passenger, it will compensate for the leak. The big risk is if you accidently bump the handle towards the release position while in psgr on a freight train, you will release the train.
Jeff
You’re right about the operation of the equalizing reservoir. I was just trying to simplify things in my explanation…
Yes, I am sure the method I described is how we made the graduated release feature on passenger equipment function. When the graduated release cap of the car control valve is set to graduated the car does not release as soon as the brake pipe increases 1.5 pounds (2 in my passenger days). As the brake pipe increases the the car dumps a portion of it’s brake cylinder pressure. When you move brake valve back to lap that stops the brake pipe increase and brake cylinder pressure decrease and closes all ports in the control valve. If there is any brake pipe leakage it will come into play and slowly increase the brake pipe reduction and hence increase the brake cylinder pressure. The idea is to make a smooth stop with the least amount of brake cylinder pressure possible. If the passenger equipment (cars) is to be moved in a freight train the graduated release cap on the control valve must be changed to direct release.
There is a limit as to how long a train graduated release can be used on to avoid rough handling, This is due to the increased length of the brake pipe and that the propagation rate of any brake pipe reduction or increase is the same physics as on a freight train. I don’t recall the exact number and haven’t found my old air brake books that are packed from our last move, but I recall being on a few 25+ car specials and trying to use graduated release resulted in some uncomfortable moments on the head end…
If we could successfully maintain the schedules and do it with smooth starts and stops 40+ years ago with the Rock Island’s antique equipment, I have great difficulty believing that it can’t be accomplished now with the near state of the art cars, locomotives and braking equipment. available. Personal opinion is that either some don’t care or they’re not being taught how to do it right.
Metra is still using coaches from the 70s and locomotives from the 80s. And, the only think new about the brake system is the blended brake, and that would make for a rougher ride as the dynamic kicks in and out and the locomotive’s brakes fade out and in.