Camera follow a rolling object without looking at it...

Thanks to Peter G's answer on "Camera follow a rolling object", I have implemented a nice and smooth camera that follows a dynamic sphere down a path. The issue I'm having is that I don't want to Look At the sphere all the time, but center my focus on the "centreline" of the path. Imagine a bobsled downhill track and a marble rolling down. I would like to make the camera follow the marble and bank when the track banks, but always look towards the middle of the track, so that the marble "moves" from side to side (relative to the screen). It somewhat resembles a camera on "railroad tracks", rolling down behind the marble. Currently I'm trying to do this by combining several dynamic objects, to resemble a bobsled following the marble (connected to the marble using a spring joint). The problem with this implementation is that the camera "sled" reacts to it's own dynamics and causes all sorts of problems (e.g. sled bumping into marble). Any ideas would be extremely helpful.

Thanks T