Car becoming uncoupled.

Welcome to the world of "lobster claw" couplers.

Nick put a rubber band around your coupler or if it has thumb tack style spring bend it up. I have done this to mine. I have not had any more problems. 

I never have this happen. I never uncouple my cars so I use small cable ties and they do not open. If it is a coupler that can not be tied shut I Super Glue the Shut. I Love Super Glue I about go crazy when the train comes apart. 

I have this problem on my son's Lionel Amtrak HHP-8 set going around O-36 curves.  Due to the forces involved on the couplers from the locomotive pulling it one way and the weight of the cars pulling another, neither of which are in line with the drawbars, they pop open.  It gets much worse when you add more cars behind them.  I twisty-tie them closed and make sure there are no cars behind the Amtrak cars on the train.

Is the thumb tack catching on the curve?  Does it always happen on the curve?  I had a passenger car where the thumb tack hung a little low and caught on a curve or turnout.   I was able to "reassemble" / adjust it slightly so it was up higher and that fixed the problem.  I didn't even have to use any tools. 

It could also be the spring in the knuckle coupler needs to be replaced or stretched slightly.

Demand better couplers or go Kadee.  I can't believe the manufacturers have not fixed this yet.  I also can't believe that 3 out of 3 replies recommend tying them together to keep them from coming loose.

UNACCEPTABLE!!!

I like pulling long trains last show I was pulling 40 cars .Most of the time more.  About 2 hrs into the show my train came apart. I thought the were are cable tied shut.  20 min later I found another one that was not tied. They are all tied now. I wish I had all non operating couplers.

Kadees are nice but when you have 100's of cars I will Accept cable ties or Super Glue

Most coupler uncoupling problems are easy to fix - just increase the spring tension. You can bend a flat spring or stretch a coil spring and that will fix at least 90% of the problems. I run 40-car trains all the time and I've never glued a coupler. I only use rubber bands or cable ties as a temporary expedient until I have time to do a proper repair. There are two exceptions to the easy-repair rule. One is the modern couplers without the thumbtack - they are held together with a rivet that has to be removed to get at the spring, and good luck finding a replacement rivet that size. The other exception is the old, cheap couplers with the one-piece plastic molding that formed the spring and the holder for the thumbtack. Those you have to replace the part, which fortunately is cheap and easily available. 

For those of you who superglue your couplers - if you ever sell rolling stock on this board, eBay, or at train shows, be honest and up front that you've glued them. I once bought a set of rare and hard to find passenger cars, only to find that a previous owner had done a very sloppy superglue job on the couplers. It took me a couple of hours to get them all unstuck and working properly. 

The only real "simple" (i.e. without taking the coupler apart) fix for those is to superglue the knuckle shut, or put it at the end of the train.

OK Nicholas, that's exception #1 in my previous post. You can't stretch the coil spring without taking the whole mess apart, which isn't easy. I'm not sure how that particular coupler comes apart, but on other ones there's a rivet that has to be drilled out and replaced.  What I would do with that one is take either some soft iron or copper wire or a black cable tie, and wrap it right behind the uncoupling tab on the coupler shank to block the rearward movement of the tab. A cable tie would probably work best on the coupler in the picture. You might need two cable ties to take up all the space behind the tab. Done just right, this will prevent the coupler from opening without messing things up with superglue. I've done this on several of the newer style K-Line trucks, which are similar to the MTH truck in your photo, and it works. Be sure you have the coupler closed before you start the operation;  if you do it right, the coupler will be locked in position when you're finished. 

Those MTH trucks are a PIA to take apart, and even worse getting them back together. If you don't plan on coupling and uncoupling, just glue them shut. Lobster claws look bad enough already, why make them look worse with O-rings or zip ties! 

Nicholas,

If the car is a premier, you should be able to drop the truck from the car and remove the offending coupler and replace it with a new one.  An MTH dealer offering repair service should have the parts.  FWIW.

I will just keep using Cable ties( can't see them on the car) or Super Glue on the ones I can't tie. We get asked at show all the time how we can sun long trains and not have our couplers come open. We have shown many people how to Cable tie them. 

Works for me.

We get a lot of the same questions when we have our large modular layout set up at shows (Independent Hi-Railers, Midwest Division). Those of us who use nothing but Kadee couplers get the most questions related to, "What are those small, realistic looking couplers on that really long train?". One of us then picks up a car, that has been up-graded to Kadee couplers, and lets the visitor look and handle them for themselves.

 If I did not have 100s of cars I would use Kadees. I think they are super and look much better. When My trains are running I never even notice the big couplers.  I do have a few cars with Kadees bought them that way what is nice they couple to the big ugly couplers. I have traded cars without Kadees for my cars with kaddees 

some of us do not do switching operations and do not like our couplers to open.

I use these when the couplers open.

http://www.amazon.com/Styling-...ssence/dp/B004J28YDK

I have one car with the same issue.  I discovered the "lock" on the bottom wasn't completely closing.  By lock I mean the spring forced piece with the nub that goes into the hole.  The nub wasn't going all the way into the hole so even small bumps could jar it loose.  Don't know if it was getting stuck going in or if it was just a weak spring, but once I started manually pushing the nub all the way up my problems dissappeard.  Hopefully your fix is as simple.

Count me as another person who became a Kadee convert.  The amount of cars you have is really not relevant, IMO.   I have quite a few number of cars myself, but I certainly didn't plunk down the money and a a bulk of Kadee couplers and convert them all at once.

I just buy them a few pairs at a time and do the conversion in the same manner.  I've gotten close to 80% of my collection converted over, and that was over the course of a year (again, just taking my time).  In the meantime, I just zip-tie or use the dental rubber bands on the 3-rail couplers until I finally get them all converted.

I am not going to put Kadees on 400+ cars. The Cable Tie fixes it fine. JMO