Editing Rail transport operations (section)

=== Signalling ===
{{Main|Railway signalling}}

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

==== Fixed block signalling ====

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 [[Railway signal|signals]]. Alternatively, [[cab signalling]] may be in use.<ref>{{cite book| title = Elements of Railway Signaling| work = General Railway Signal Co. | url = https://books.google.com/books?id=LjIvnQEACAAJ | date = June 1979| publisher = General Railway Signal Company }}</ref>

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.

==== Moving block signalling ====
{{Main|Moving block}}
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 directly to the trains. It has the advantage of increasing track capacity by allowing trains to run much closer together. The system is only used on very few independent networks such as underground lines.{{Why?|date=September 2023}}