D, where the process is repeated.) A separate track was typically used to haul the empty cars back to the top. The original implementation of this type of system is credited to the Mauch Chunk Switchback Railway, which hauled coal and passengers from 1827 until 1933. This was very popular with tourists, and led to the development of the roller coaster. Self-acting incline In the UK and elsewhere, a self-acting incline is one in which the loaded wagons going down pull, via a cable and drum, the empty wagons going up. There might be two separate tracks, or a single track with a passing loop. This system was widely used on slate railways in Wales. A variation on this system is the cliff railway for passengers, for example the Lynton and Lynmouth Cliff Railway. Both passenger cars are equipped with water tanks and, at the start, both tanks are full. Water is then let out of the tank on the lower car until the difference in weight between the two cars causes them to move. Examples United Kingdom The Ffestiniog Railway in Gwynedd, northwest Wales, was built in 1832 to carry slate from quarries high in the hills to the sea at Porthmadog. The line was laid out for the wagons to descend by gravity, while horses were originally used to haul the empty wagons up the hill. On the downward journey the horses travelled in a Dandy waggon at the rear of the train. Later on, steam haulage was adopted. This narrow gauge railway is still operational but all passenger trains are now locomotive-hauled. Demonstration gravity trains are still occasionally run using original wagons up to 50 at a time. United States In the United States, The Delaware and Hudson Canal Company operated an extensive gravity railroad system from 1828 until 1898. With 22 separate lift planes, the purchased in 1886 by the recently constructed Shohola Glen Summer Resort (1882) and used until 1907. Due to the success and advancement of the gravity railroads, a second gravity operation at Hawley and Pittston was created in 1850. This route from Port Griffith (Pittston) to Paupack Eddy (Hawley) allowed Pennsylvania Coal Company to directly ship anthracite from its Northern Coal Field mines in the Wyoming Valley to Delaware and Hudson Canal and ultimately to the New York markets. The Ontario and San Antonio Heights Railroad Company was a railway in Ontario,