Thanks.  After some time watching the cars go back and forth before and after the curve, I found the one car that seems to be causing most of the problems.  I took the coupler out of the mounting pocket and found some flash on the cover piece that I think was binding the movement of the coupler.  Also, the one piece of straight track before the curve is warped slightly and the cars actually run down hill at slow speed so there is some slack between the cars before and after the curve.  After cleaning the flash, it didn't uncouple as often but it still uncoupled.  I will try to clean it up a bit more, it is kind of like doing micro-surgery because I have to use a jewlers loupe to see the pieces!  I will try to mount the track to hold down the bump and see if that fixes the problem permanently.  The couplers are dummy couplers and not like the ones on the HO or O gauge cars I have.  Is there a better coupler for N scale that won't kill the yearly train budget? The track is the ez track but we plan on using flex track for the permanent layout.  I do thank you for the help, my son is a happy camper and we have alot of fun together in "dad's train room" as he calls it, I think the train bug bit him!

Sorry, I forgot to add this.  I did check to see that the couplers move freely in the mounting box and they do seem to move fine.  I even tried to loosen the mounting screw 1/4 turn but that didn't help, it just made things worse.

I am using the curves that came with the set which I believe are 11" radius (?).  They are just starter sets with knuckle couplers.  I thought about the couplers being out of alignment and already checked that and they are fine (at least seem to be to the naked eye).  The one set that has no problems is the Union Pacific Highballer set with the F3 engine.  The one that is having the problems is the Santa Fe Thunder Valley set.  At first, it was just the boxcar that had the problem so I switched it's position to make sure that was the car with the problem but now, all the cars are doing the same thing except, whatever car is first behind the engine, seems to stay coupled to the engine.  I also thought possibly it was a problem with the track being off level on the one curve but it happens on all curves no matter what speed.  I do appreciate all the help as I have a special needs son who is 4 years old and is getting frustrated and I don't want to spoil his enthusiasm on the hobby.

I have collected and run O scale all my life and I am new to n scale so I don't exactly know what I am looking for as what the problem could be but it seems that one of the sets I got my son is having problems.  We got two n scale sets and an ho set.  The problem is that it seems that the couplers don't want to follow the truck as it goes around the curve.  The other set has no problem on the same track.  I tried to loosen the screw a 1/4 turn that holds the coupler but to no avail.  Any ideas?