Hi Bernd

My VT 11.5 is the Märklin 39605 (issued in 2002).
For sure the cars open by removing the 2 screws underneath between the 2 bogies and like the motor units separate in 2 parts BUT:
- often long sealed items remain glued because assembled while the pain was fresh
- be prepared to deal with the complex elongation mechanisms and fragile couplings
- have a camera ready to take detailed pictures so that if a part "flies" or if a wire breaks you will know how to repair it
- don't push too hard on the sides of the car as it will unglue the windows

Here is the exploded view diagram


Cheers
Jean