If a cut of cars is being

If a cut of cars is being pulled across a spot where there is a powerful under-the-track permanent magnet uncoupler you can get undesired uncouplings if there is a 'stutter' in the train when a set of couplers are just above the magnet. All it takes is a momentary lapse of tension in the cut and those knuckles can open up.  If you have steel axles on your wheel sets, the magnets can grab these, trying to pull each adjacent set of cars together as they pass over the magnet (if it's a really powerful magnet) causing slack in the couplers.

The only way to fix this is, to not have the strong magnetic fields present all the time.  Two alternatives (perhaps there are others):

1) Use an electro magnet uncoupler. These are only active while you're pushing a button.

2) Mechanically move a permanent magnet up and down under the roadbed. In the up position is nestles against the bottom of the tracks. In the down position it's an inch or more below the track and the magnetic field at track level is too weak to have an effect.

Cheers,

Charlie

Uncouplers

"Perhaps removing the metal back plate from the magnet would be useful"

From this I assume it's accessible.  I'd try that, as  a "pole piece" focuses the magnetic field.

If you get a change from 'too effective' to 'not effective', try creating a pole piece of your own design.  I'd try to spread the field (make it wider) across the rails, or lengthen the field along the rails.  Since it's hard to see magnetic fields (yes, you can use iron filings, but that is messy), my suggestion is to alter size & shape of the plate. (First, try rotating the plate you have 90 degrees).

Be sure to use soft steel (not stainless) that is easy to attract to a magnet.. different steels can vary in effectiveness.  And you CAN bend it, too.   Perhaps a coat hanger wire can be used as a start.

Check: Are your tracks sloped toward or away from the uncoupler?   At what speeds does this occur?

I'm just hoping you have only a  few such locations, as it can be time consuming. 

There is a guy on another forum that makes ho scale

electromagnetic uncouplers from Kadee O-scale electromagnetic uncouplers.  You can find it at   http://www.the-gauge.net  if you go to the index page and then go to the forum titled "Engineer's Technical Workshop" there is a thread titled "Electromagnetic Uncoupling That Works."  Right now it is the second thread from the bottom of page one, but it may migrate to the second page if there are a couple more threads started there.  He starts with a Kadee O-scale electromagnetic uncoupler and modifies it for use with ho scale.  The resulting magnetic uncoupler is strong enough that you need to eliminate all steel from your cars except the Kadee trip pin. 

I copied the url to the index exactly, but it keeps coming up with an error 404 code.  If you type that url in the "search box" on your search engine, it will open, but I'm not getting the link to work from this post.

Shorten the length of the uncoupler

To avoid unwanted uncoupling, especially with metal axles, I use kadee uncouplers that have been cut cross wise into 3 short pieces with tin snips.(Three uncouplers for the price of one.)These work well with the no. 58's I use. However you do need to spot your couplers  more accurately.

Cheers,Steve

Add drag?

Dear Doug,

Kadees have a tang on the inside of the knuckle, such that it takes some slack to allow them to "pop open" over a coupler. As has been suggested, if the train "jiggles" or hesitates over a uncoupler, then that may give just enough slack to allow the deed to occur.

Now, smooth running locos, and smooth train-control on the part of the operator helps,
but for my shelf switcher show layouts, where "unexpected uncoupling" is to be eliminated at all costs,
adding 'drag devices' to each car has been the solution.

Now, don't get the wrong idea, we're not looking to stall the car. We're just looking to the advice given by Kadee themselves, esp RE N scale cars, where Kadee themselves _provide_ little coil springs to go over the pin-point of one axle on each car, thus giving enough drag to ensure "constant tension" on the train.

In HO, such axle coilspring drag devices seemed impractical. However, a online page showing how a modeller used Kadee #5 centring springs to add pickups to a steam loco tender appeared to provide a solution.
(I wasn't concerned about electrical pickup, although it's a useful tool in my "modelling bag-of-tricks",
just the slight-increase in rolling resistance)

Put simply 
- bend the thin side "spring leaves" to a 45degree angle
- flatten out the 'bent one'
- trim back the flat base of the spring, maybe 1/2 way thru the large pivot hole
- now sit the assembly on top of the truck bolster,
(between the central bolster pivot and one of the sideframes)
such that the angled "spring leaves" rub against the back of the wheels
- and glue or pin as required to anchor in place
(In the examples below, I use PECO SL14 trackpin/trackspike as an anchor pin,
you can see the round domed head)

One of these per car,
(IE you're only adding a drag device to one truck on each car, not both),

will cut the "how far it rolls with a slight finger nudge" distance to maybe 1/2 or 1/3 of what it was,
which is enough drag to help keep the couplers together,
but not so much as to measurably affect the pulling power of the loco,
(my stock Athearn SW1500s, Kato NW2, and Atlas Alco S-series switchers will still waltz away with as many of these cars on behind as I have track-length enough to place them on, have tested 15+ cars with no slippage,
and I'm one of the guys who uses graphite on my rails... ),

or the usable rolling qualities of the car itself.

It's made my shelf layouts a pleasure to operate,
as opposed to a constant agravation due to "spontaneous uncouplings"...

Just a thought...

Happy Modelling,
Aim to Improve,
Prof Klyzlr

PS No, it does not interfere with the operation, tracking, or minimum radii performance of the car

Inadvertent uncoupling

thank you to all of you for giving suggestions for fixing my problem. Some of these suggestions would never have occurred to me without your thoughtful inputs. Isn't it great to have a forum such as this to solve these dillemas. Thanks again to all. Special thanks to Pro Klyzir...those are excellent photos you included with your suggestions...worthy of any magazine's photography.thanks for all the effort in replying to my uncoupling problem.

Doug Dyer

Link correction - electromagnetic uncoupler

My two cents - link to the blog on O scale uncoupler modification referred to by Russ above:

http://www.the-gauge.net/forum/viewtopic.php

The problem with the link Russ supplied is that there were two spaces between the "net" and the train "/".

 Edit: P.S. - You will need to register on that site to view the pictures.

Dear Doug, Thanks for the

Dear Doug,

Thanks for the pics feedback. Not sure they're "publishing quality", but they get the concept accross

Just remembered one other benefit, 
some modellers express the want to emulate the "slack runout/bunching up" of prototype train handling.
With the slight drag the above system provides, even short trains can have visible "one car at a time" slack runout when starting...

Happy Modelling,
Aim to Improve,
Prof Klyzlr