The very top part is glued to the next one below. It has to be carefully cut of or using a very little saw to get it of the part below. Disconnect the connector for the lights and unscrew the four nuts.

Disconnect the connector for the lights and unscrew the four nuts. The plastic part holding the train and the gear can be removed. Work one level down after the other. After removing the nuts the levels are glued to the next base so the glue needs to be cut or removed as well.

View at the very base level with the motor on the left and the gearbox open.

Found the problem to be a stuck gearbox which prevented the trains from moving

Found problems on the PC board as well. Two transistors were overheated and not working any longer due to blocked gear box.