impossible roller coaster animation?

Hello everybody,
I would like to present to you an animation problem which attracted some interest in the German “blendpolis” forum (I was surprised) and for which a satisfying solution has not been found yet (I was surprised again). I had not expected the problem to be demanding for experienced blender users.
Here is the task - hopefully my English will do for explaining the main difficulties:
I would like to make an animation of a roller coaster train, camera not riding with the train. My main interest is the movement, not a detailed modeling work.
There are different characteristics that I would like to give that train’s movement in order to make it look realistic:

  1. The train should consist of more than one wagon (let’s say several cubes).
  2. The train should keep it’s total length (the wagons are connected to eachother in a real roller coaster).
  3. Accelerating / slowing down should be possible for the complete train without affecting it’s length.
  4. The rotation of each individual wagon around every axis should depend on its individual position on the track.
  5. It should be possible to use a track that twists, climbs, leans in every dimension, like the one of a real roller coaster (=not just a flat track as for a normal railway train).
    Different approaches (downloadable .blend files) for an animation of this kind can be found here (including a very interesting one by user “qp”, making use of a curve deform modifier):
    It seems to be extremely difficult to have all the characteristics mentioned above combined at the same time. I am not sure any more that there is a clean solution with blender - even if the movement as such does not seem that extraordinary to me!
    I would be very glad to read about your ideas/experiences/questions, thank you in advance.
    Greetings from Freiburg, Germany

I have found a solution for this animation problem.
See my last .blend attachment in the thread of the German blend.polis forum: