Go to the Meanstreak threads on PBuzz for a good explination of this. Just read RideMan's posts... on anything really, he's brilliant.
Basically, without the trims the train has enough speed to interact with the track the way it was designed to. A good example is the first banked turn. Here's a simplified explanation. The train should be going fast enough that it is forced onto the outside track, basically holding it in place. The guide wheels stay in contact with the side of the track. At the lower speed it has now, it shuffles... basically going on and of that rail and shaking the train.