If the violet tank moves in a curve, like in the picture anti-clockwise, the blue box always hinges to the outside. I would have expected the opposite, that a object on a joint would lean to the inside of the curve?
All you've done is added a soft pin joint to pull the trailer towards the tank. In regards to the floor, it's completely frictionless. With no friction to pull the trailer back behind the tank, it will fly away from the center of the circle.
You need to add extra joints like the tank has holding it to the control body. Though because the trailer is not powered, you'd simply attach the joints from the trailer to the static body.
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