@terry: thanks, i haven't had a problem with uncoupling, just derailing.

@jward: Yes, one end of the gondola has the horn hook coupler and the other end has none so it would be a good connector car.  A train show would be excellent but i think i've missed the ones in ottawa this year.  There's quite a train-focused hobby shop in ottawa and i'm sure they'll help me out now that i have more of a clue.  I note too that a web site called "central hobby" has reasonably priced roundhouse caboose's already assembled. They come with kaydee couplers but i can put one on the back of the gondola and have a complete-looking little outfit.   

http://www.central-hobbies.com/products/roundhouseho.html#NECC

So, an update:

I carefully aligned a circle track and glued it down with a few dabs of latex caulk at strategic points.  I cleaned the track carefully and this eliminated all the basic issues so i could see what was going on.

The derailing seems to be due to the couplers.  All the cars have the basic horn/hook couplers but the engine has them frame mounted while the inexpensive cars from the set have them mounted on the trucks.  The mismatch seems to drive the lightweight cars off the rails almost immediately.  Weighting them helps a bit but sooner or later they pop off.  I have one hopper car that i must have bought separately.  It has the same horn/hook but body mounted and it tracks perfectly behind the engine.

So, I think I need to put the cars from the set aside and buy another good quality caboose or passenger car to finish off a little train and call it good.  I can look at replacing the body mounted couplers but maybe that's a secondary issue.  I'm pretty sure now i can successfully put up a little layout and get my grandson involved at christmas.  At 5 he's too young for it but it may fill an idle hour and spark an interest for him.

Thanks for the help and suggestions. Any further advice also appreciated.

First image shows the locomotive, the good quality open car, and the caboose from the set. Second image shows the couplers of the locomotive and the better quality car.  Third image shows the caboose coupler.

@jward: good tips.  I did not have the track fastened down and i did not look carefully for causes - i will do so.

Thanks for the responses. 
@jonathan: I'll try some weights.
@infamouselijah: it's the basic bare metal track.  I see the new ez track on amazon but  i don't think it would mix with what i've got and it seemed quite expensive.  I might have a look through a thrift store and see if i get lucky.

Maybe i could buy a new tender if the cars and connectors have improved.

when my kids were young we had a CN Hustler set up on a 4X8 in the basement.  It's been packed away for 25 years but with a 5 yr old grandson coming at christmas I figured I would get it out.

This is a basic set with locomotive plus 3 cars and there are a ton of extra tracks stored with it - i think we had a basic figure 8 with a flyover.

I set up the 36" circle and got the locomotive running well enough.  I can not get any of the cars to stay on the track behind it though.  It seems like the cars are so light that the spring of the connectors is enough to push them off the track on a curve. Weighting the hopper car seemed to help a bit but it still came off after a few minutes.

So: clearly i need to clean the tracks and maybe buy new joiners.  Is there anything else that would help with the derailing?  Any other general tips?