We might be able to help you a little bit more if you gave us some more information:
1. Does the train always derail at the same place, if so where on the track is it, curve, straight, switches or transition between straight and curve?
2. Is it the engine or the cars that derail?
3. Are you using the K-27 (Bachmann or Accucraft)or some other engine, if it is another engine what is it and who made it?
4. Whose track and switches are you using? 7'diameter is not a common commercial curve. LGB's diameters are 4', 5', and 8'. I had to replace all my LGB 16000 switches with Aristocraft large radus switches (10' diameter) when I got my Accucraft K-27. It would not go through the 16000 series switches. I am now in the process of replacing all my switches with LGB 18000s (17' diameter). Fortunately, I got all I needed before LGB went the way of the Dodo bird.
5. Is the switch coming out of the reversing loop thrown automatically or are you relying on the engine to push it over? My experience with switches is that only LGB steam engines will routinely go through and open closed switches. The leading trucks on other manufacturer's (Aristocraft, Accucraft, Bachmann) steamers will derail.
Unless you are running battery or live steam, I would get rid of the reversing loops and run a parallel track, sort of a dog bone shape. Reversing loops outdoors can be a real pain in the neck.
I think that some people have talked about running K-27s on 8'diameter curves (all of my curves are 10' diameter), I don't recall anyone mentioning that they had run it on tighter curves. All of the Bachmann K-27 drivers are flanged. There is a limited amount of side play in the drivers. Therefore they might ride up and off of the rails if the 7' is too tight. If it is the engine and it always derails at the same place you should get down on the ground and run the engine slowly through that section of track and carefully watch it. That is something that I have had to do occasionally. It took me two years of fiddling before my K-28 would go around my track once. Now it runs like a champ.