it is my assumption that when you talk about the 18" or 22" radius you are talking about the curve??  If that is what you are talking about the engine will derail on the straight section also.  with or with out a load.

this set can be found on the bachmann website.

http://www.bachmanntrains.com/home-usa/products.php?act=viewProd&productId=4

I would assume that the track that was sent with the train in the kit would be the correct track..

For Christmas this year I was given an Overland Express Limited train kit.  I went to set it up this morning after weeks of preparing my room for it.  I have installed a perfectly flat surface for this kit.  After getting the train connected on the track it derails almost immediately.  9 out of 10 times it will derail the minute it hits the section of track with the power connected to it. I have tried this with the cars attached to the engine,  just the engine.  There is also no common factor One time it will be the front wheels  the next it will be the back wheels.   Also the cars will not stay coupled together.  Any help or ideas you might have to this issue would be greatly appreciated.