Firstly: your previous post: the ‘smashing together’ of your cabin is because, when you animate a group, every single member of the group is animated (given a pivot etc…). So all the meshes forming your cabin where drawn together with their pivots at the same location on the path curve.
You should rather make a Block (Rhino’s Block command) of composite object. The block then when animated only gets a single pivot.
Your attempt on the simplified version is deservingly.
Setting the trailer as child of the cabin in your ‘trailer 001.3dm’ model doesn’t work mainly because parent child relations are based upon the object’s base-positions, i.e. their position apart from any animation. In the left figure below you see your objects as such. In order to get things working at least the various objects must be in there ‘logical’ mutual place (right figure).
This model Track 001.3dm (96.5 KB) shows you a way of setting up the proper IK-chain.
Controlling the speed and movement of a constrained-to-path-object can be done by adding keyframes defining the ‘parameter’ of the constraint. An example: Track.3dm (59.7 KB)
Good luck,
Luc