Natural Monopoly. A natural monopoly exists if an industry can only support one firm.
An example of a natural monopoly is London Underground. Only one set of railway lines can be laid so only one firm can operate them.
Natural monopolies usually exist in the rail and utility industries. A large start-up investment in the infrastructure is required before a firm can operate for example, railway tracks, gas lines, water pipe networks and the electricity grid. This initial investment is very costly so fixed costs are very high. AC and MC begin high then continue to fall. AC remains above MC. A natural monopoly will produce at MR = MC and make super-normal profit.