It’s a matter of mechanics. In order to follow a 3 dimensional track the hinges should be able to evolve 3 dimensional. If not the train is vertically stiff as a stick.
In BongoIK_03 001.3dm (78.9 KB) I made all hinges “Universal” allowing rotation along Y and Z axes – and the IK-solver finds a nice solution. 2 rotational axis suffice to give the joints enough freedom. Notice there is no telling how IK will make the chariots turn around their x-axis. When you add freedom of rotation around the X-axis you’ll see the chariots even twist more. But then again ‘mechanics’ … in order to control the vertical position of the chariots a second rail is required.