The first trains were rope-hauled, gravity powered or pulled by horses, but from the early 19th century almost all trains were powered by steam locomotives. From the 1910s onwards, steam locomotives began to be replaced with diesel and electric locomotives; although these new forms of propulsion were far more complex and expensive than steam power, they were less labor-intensive, and cleaner. At about the same time, self-propelled multiple unit vehicles (both diesel and electric) became much more widely used in passenger service. Dieselisation of locomotives in day-to-day use was completed in most countries by the 1970s. Steam locomotives are still used in heritage railways operated in many countries for the leisure and enthusiast market.