Railroad operations

A railway has two major components: the (the locomotives, passenger coaches, freight cars, etc.) and the infrastructure (the permanent way, tracks, stations, freight facilities, viaducts, tunnels, etc.).

The operation of the railway is through a system of control, originally by mechanical means, but nowadays more usually electronic and computerized.

Signalling systems used to control the movement of traffic may be either of fixed block or moving block variety.

Most blocks are 'fixed' blocks, i.e. they delineate a section of track between two defined points. On timetable, train order, and token-based systems, blocks usually start and end at selected stations. On signalling-based systems, blocks usually start and end at signals. Alternatively, cab signalling may be in use.

The lengths of blocks are designed to allow trains to operate as frequently as necessary. A lightly used branch line might have blocks many kilometres long, whilst a busy commuter railway might have blocks a few hundred metres long.

A disadvantage of fixed blocks is that the faster trains are permitted to run, the longer the stopping distance, and therefore the longer the blocks need to be. This decreases a line's capacity.

With moving block, computers are used to calculate a 'safe zone', behind each moving train, which no other train may enter. The system depends on precise knowledge of where each train is and how fast it is moving. With moving block, lineside signals are not provided, and instructions are passed direct to the trains. It has the advantage of increasing track capacity by allowing trains to run much closer together.

Most rail systems serve a number of functions on the same track, carrying local, long distance and commuter passenger trains, and freight trains. The emphasis on each varies by country. Some urban rail transit, rapid transit and light rail systems are isolated from the national system in the cities they serve. Some freight lines serving mines are also isolated, and these are usually owned by the mine company. An industrial railway is a specialized rail system used inside factories or mines. Steep grade railways are usually isolated, with special safety systems.

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Wikipedia