Fridge Logic: The incompatible couplings, and the bodged solution with rope form a plot point. But it would have been possible to join the coupling chain from the flatbed/coach straight onto the chain on the locomotive.
The chains from the coach were being used to hold the body of the coach to the flat bed, Mess around with them and the coach could come off the truck at the first corner. Plus the man who used the rope to couple the tender to the flat bed said that, given time, he could rig something up that could pull "The Queen Mary" (The Ocean liner). The others lament that they haven't the time for that
The bus company's plan to wreck the train falls under this. Even if they had succeeded in getting the line closed, they'd have been screwed when British Railways turned up to reassign the engine to another line.