Thank you very much for the positive and encouraging feedback! The idea with custom built bogies and gearing instead of the train motor is brilliant. I redesigned the model and was incredibly happy when I realized, that the new and much lower bogies allowed me to simplify the lower part of the body a lot. In the first version I tried to hide the upper part of the train motor behind a skirt of tiles. This was the only solution I found that allowed enough freedom of turn for tight curves. This is now no longer necessary.
So far I don‘t know whether the new arrangement will give a reasonable speed and whether it will derail easily or not. I‘m a bit concerned, because the vertical axle that brings the motion from the motor to the driven axles acts as pivot for the deflection of the bogie. But as I‘m obviously not the first and only one who uses this arrangement I will give it a try.
So, I wait for the missing parts to arrive and look forward to build this MOC for real.