The main line's primary purpose was to carry finished slates and Fullersite (powdered slate) to Port Penrhyn where they were loaded onto ships or, from February 1852 when the Chester and Holyhead Railway opened its Port Penrhyn Branch, onto standard gauge trains. The ships and standard gauge trains took the products to national and international markets. Secondary traffic consisted of slate materials which were finished at workshops at Port Penrhyn into bitumen blocks and, notably, writing slates on which the port had a British near-monopoly in the 1930s. Paying backloads were few, but sufficient coal was brought from the port to the quarries to warrant the company buying a fleet of 50 coal wagons. The line also carried internal user traffic such as spare parts. The intermixing of standard and narrow gauge lines at the port produced some remarkable pointwork.