"Attempting a running release at very low speeds may damageequipment, lading, or track."
WHY?
On a conventional train (no distributed power, all engineers on the head end) the brakes release from the front to the back. On a long train, the front end can be released while brakes on the rear end are still applied. As the head end starts moving faster than the rear end the slack can run out, either because the engines are pulling or maybe just "drifting" in idle, it can cause stress on the draft gear. Slow speeds increase the braking effect, that's why really low speed releases are usually frowned upon. If you're lucky you just get a knuckle. If you're aren't lucky, you get a drawbar. If you really aren't lucky, you get a wrong-end drawbar. (Drawbar probably isn't the correct term, but it's what we use.)
Besides speed, there are other things to take into consideration when deciding to make a running release. Things like train length and make up, physical characteristics of territory, etc. Some times you are better off just coming to a stop before attempting a release.
Which railroad's air brake and train handling book are you looking at?
Jeff
Thanks for answering this one, Jeff. Am I right in saying that this applies particularly if you were planning to stop the train and then determined that it was not necessary to come to a complete stop?
You mention really slow speed releases; if you have been running fast and had to use air to reduce speed for a curve, I presume that it is safe to release the brakes once the curve is past.
Incidentally, I believe that I have seen a pulled lung (drawbar); it was not a pretty sight, lying on the ground--and I think it was a wrong-end.
Johnny
Probably most really slow speed releases would be in cases where you were approaching a signal requiring a stop. Right before you stop it clears up. Other times it could be you misjudged where you wanted to stop and set air too soon. (I've done that a time or too, myself. Most times It's still better to stop too soon and pull up rather than stopping too late.)
The other day I added to my collection a 1961 air brake and train handling book from the Rock Island. (Also a 1951 CB&Q rule book.) I have a few other RI ABTH books from different years and a couple of MILW ones. I've noticed comparing the modern era RI, MILW, and the current one I carry to those from years past, that the older ones are more technical and sometimes tell you why you need to do something instead of just telling you to do it. That 1961 book when talking about controlling slack says that the braking force increases as speed decreases. From the book, "The braking force of air brakes at 20 mph is approximately twice that at 60 mph, and at 1 mph it is three times as that at 60 mph for equal brake applications."
So using air at higher speeds to slow down for a curve or speed restriction shouldn't be a problem.
Jeff,
Is it possible that the 1961 rule book was assuming cast iron brake shoes as opposed to composition brake shoes? One advantage for the latter is braking force is more even across the speed range.
Interesting stuff.
- Erik
That's possible, but just from experience (feeling the train "tugging" during an application or watching the load meter) the effect still seems to hold true. Feeling the slack being pulled out is more noticeable at slower speeds.
I'm sure there are other reasons too, like if the cars are loaded or empty, have long travel (cushioned) drawbars, etc for feeling the slack adjust. Still it just seems like I feel it more at slower speeds when making the first brake application.
Thanks!
I am not surprised that that the braking effectiveness increases at low speeds as the wheels and shoes would be cooler than at higher speeds. I do know from reading about runaway trains that braking effectiveness really takes a dive when the wheel tread temps reach ~600 degrees (F).
I would guess that the increase in braking effectiveness at low speeds could lead to the wheels locking up and causing flat spots.
P.S. On a related not, the cut-off wheel style rial saws work a lot faster when the cutting point is moved back and forth across the rail head as red hot metal is softer and doesn't abrade as well as cooler steel.