Why are large ships called liners?

Regular scheduled voyages on a set route on the sea are called ‘line voyages’ and vessels (passenger or cargo) trading on these routes to a timetable are called liners. In older usage, ‘liner’ also referred to ships of the line, that is, line-of-battle ships, but that usage is now rare. Today, the term refers to a ship that is constructed to a higher standard than a normal cruise ship, enabling it to cross oceans such as the Atlantic and Pacific with passengers, in inclement weather conditions.