"Tram line bruises" are caused due to injuries caused by

When a person is stuck with a cylindrical object such as iron bar, the bruise pattern formed is distinct. The skin surface is indented and blood vessels at the edges are ruptured. Blood is squeezed out of any vessels along the point of contact, but the vessels remain relatively intact (paicularly if the suppoing tissues are lax). When the impacting object is removed, blood flows back into the undamaged vessels, but leaks from the damaged ones. The resulting bruise is termed a 'tramline bruise' because it appears as a pale linear central area lined on either side by linear bruising. Ref: Krishan Vij textbook of forensic medicine, 5th edition pg : 218