Editing Centralized traffic control (section)

==Development and technology==
[[File:PCPOST Train Dispatcher.png|thumb|[[Penn Central]] Southern Region (Columbus Division) Train Dispatcher controlling train movements at the CTC "B" board in [[Columbus, Ohio]]. At this position, one person could handle about 25 through train movements a day.]]

The ultimate solution to the costly and imprecise train order system was developed by the [[General Railway Signal]] company as their trademarked "Centralized Traffic Control" technology. Its first installation in 1927 was on a 40-mile stretch of the [[New York Central Railroad]] between Stanley, Toledo and [[Berwick, Ohio]], with the CTC control machine located at [[Fostoria, Ohio]].<ref>General Railway Signal Co. "Elements of Railway Signaling." GRS pamphlet #1979 (June 1979)</ref> CTC was designed to enable the train dispatcher to control train movements directly, bypassing local operators and eliminating written train orders. Instead, the train dispatcher could directly see the trains' locations and efficiently control the train's movements by displaying signals and controlling switches. It was also designed to enhance safety by reporting any track occupancy (''see'' [[track circuit]]) to a human operator and automatically preventing trains from entering a track against the established flow of traffic.

What made CTC machines different from standard interlocking machines and ABS was that the vital interlocking hardware was located at the remote location and the CTC machine only displayed track state and sent commands to the remote locations. A command to display a signal would require the remote interlocking to set the flow of traffic and check for a clear route through the interlocking. If a command could not be carried out due to the interlocking logic, the display would not change on the CTC machine. This system provided the same degree flexibility that the manual traffic control has before it, but without the cost and complexity associated with providing a manned operator at the end of every route segment. This was especially true for lightly used lines that could never hope to justify so much [[overhead (business)|overhead]].

Initially the communication was accomplished by dedicated wires or [[twisted pair|wire pairs]], but later this was supplanted by [[Pulse-code modulation|pulse code]] systems utilizing a single common communications link and relay-based telecommunications technology similar to that used in [[Crossbar switch#Telephone exchange|crossbar switch]]es. Also, instead of only displaying information about trains approaching and passing through [[interlocking]]s, the CTC machine displayed the status of every block between interlockings, where previously such sections had been considered "[[dark territory]]" (i.e., of unknown status) as far as the dispatcher was concerned. The CTC system would allow the flow of traffic to be set over many sections of track by a single person at a single location as well as control of switches and signals at interlockings, which also came to be referred to as '''control points'''.<ref>{{cite web |last=Calvert |first=J.B. |title=Centralized Traffic Control |url=http://mysite.du.edu/~jcalvert/railway/ctc.htm |date=1999-05-29 |archive-url=https://web.archive.org/web/20210419050858/http://mysite.du.edu/~jcalvert/railway/ctc.htm |archive-date=2021-04-19}}</ref>

CTC machines started out as small consoles in existing towers only operating a few nearby remote interlockings and then grew to control more and more territory, allowing less trafficked towers to be closed. Over time, the machines were moved directly into dispatcher offices, eliminating the need for dispatchers to first communicate with block operators as [[wikt:middlemen|middlemen]]. In the late 20th century, the electromechanical control and display systems were replaced with computer operated displays. While [[Integrated Electronic Control Centre|similar]] [[signaling control]] mechanisms have been developed in other countries, what sets CTC apart is the paradigm of independent train movement between fixed points under the control and supervision of a central authority.

===Signals and controlled points===
[[File:Uprr block signals.jpg|thumb|CTC automatic block signals along the [[Union Pacific Railroad]] [[Yuma, Arizona|Yuma]] Subdivision, [[Coachella, California]]]]

CTC makes use of [[railway signals]] to convey the dispatcher's instructions to the trains. These take the form of routing decisions at controlled points that authorize a train to proceed or stop. Local signaling logic will ultimately determine the exact signal to display based on track occupancy status ahead and the exact route the train needs to take, so the only input required from the CTC system amounts to the go, no-go instruction.

Signals in CTC territory are one of two types: an '''absolute signal''', which is directly controlled by the train dispatcher and helps design the limits of a control point, or an '''intermediate signal''', which is automatically controlled by the conditions of the track in that signal's block and by the condition of the following signal. Train dispatchers cannot directly control intermediate signals and so are almost always excluded from the dispatcher's control display except as an inert reference.

The majority of control points are equipped with remote control, power-operated switches. These switches often are ''dual-controlled switches'', as they may be either remotely controlled by the train dispatcher or by manually operating a lever or pump on the switch mechanism itself (although the train dispatcher's permission is generally required to do so). These switches may lead to a [[passing siding]], or they may take the form of a [[crossover (rail)|crossover]], which allows movement to an adjacent track, or a "turnout" which routes a train to an alternate track (or route).