A trailer is defined as: "Any vehicle or object on wheels and having no motive power of its own, but which is drawn by, or used in combination with, a motor vehicle. It shall not include a pole dolly or pole dickey, so called, nor a pair of wheels commonly used as an implement for other purposes than transportation, nor a portable, collapsible or separate two-wheel tow dolly limited only to the purpose of transporting or towing a registered vehicle." It also does not include farm machinery or implements when used in connection with the operation of a farm or estate, nor any vehicle when towed behind a farm tractor and used in connection with the operation of a farm or estate.