Direct traffic control
Encyclopedia
Direct Traffic Control is a system for authorizing track occupancy used on some railroads instead of or in addition to signals
. It is known as "direct" traffic control because the train dispatcher gives track authority directly to the train crew via radio, as opposed to through wayside personnel via telephone or telegraph, as in train order
s.
In DTC, controlled tracks and sidings (those requiring authority from the train dispatcher
to occupy) are divided into pre-specified blocks. In addition to being listed by milepost in the railroad's timetable
, block limits are delineated by conspicuous signs along the tracks. Every portion of controlled track belongs to a block, as blocks are laid out back-to-back along the entire length of the rail line.
For example, a 30 miles (48.3 km) length of main line track may be divided into three blocks, Anna, Bess, and Cloy, each 10 miles (16.1 km) long. At milepost 10, there will be a sign displaying the end of Anna block and the beginning of Bess block, and a similar sign between Bess and Cloy. A train authorized in Anna through Bess blocks must stop before Bess block ends and Cloy block begins.
Lengths of DTC blocks vary but usually take about 10 minutes to traverse. However, where the main track is paralleled by a controlled siding
, the main track block is the length of the siding. The siding is a considered a separate block. In DTC, most sidings are considered controlled track and must not be occupied without authority from the train dispatcher. Sidings are considered blocks in the same way as the main track; in fact, from a technical standpoint, very little differentiates a siding from a main track in DTC.
For example, a length of track may consist of three blocks. Anna, Bess, and Cloy. There is a siding at Bess, and this siding is specified as Bess Siding. Both Bess and Bess Siding blocks extend to the switches at either end of the siding. Anna and Cloy blocks begin on the other side of their respective switches. Neither Bess nor Bess Siding blocks may be occupied without authority from the train dispatcher.
In this example, a train may have authority straight through from Anna through Cloy. Or, it could have authority in Anna through Bess Siding, in which case it must throw the switch between Anna and Bess and enter the siding--this will usually be done to clear up the main track in preparation for an opposing train to pass. Then, it might get authority from Bess Siding through Cloy, in which case it will throw the switch between Bess and Cloy and reenter the main track.
While most DTC blocks are laid out back-to-back, the blocks actually do not touch at sidings. Instead, there is a "gray area" over the siding's switches, and the main track's and siding's DTC blocks begins at approximately the clearance point of the switch. The area over the switch must not be occupied unless authority is granted in blocks on both sides of the switch. For example, a train authorized only in Anna block must stop at the End Anna Block sign, which is a short distance from the switch points. A train authorized in Anna through Bess or in Anna through Bess Siding may proceed past the End Anna Block sign, over the switch, and then past the Begin Bess (or Bess Siding, if appropriate) Block sign.
Blocks are typically named after a distinguishing feature within the block, such as a station, town, or river.
In modern implementations, dispatchers rely on computerized systems to keep track of trains that have received authority. Typically, the computer will prevent the dispatcher from giving two trains authority over the same track. The computer system generally displays a highly simplified mock-up of the track, displaying the block limits and sidings. Track occupancy is displayed via bold or colored lines overlaying the track display, along with tags to identify the train (usually the number of the lead locomotive).
To receive or change a train's authority, the train dispatcher communicate by radio with the train crew and gives "mandatory directives" authorizing occupancy in a specified number of blocks. A member of the train crew, usually the conductor, copies the dispatcher's mandatory directive onto a prescribed form. The crew member will repeat the directive back to the dispatcher, who monitors the repeat for any errors. If the crew member correctly repeats the mandatory directive, the dispatcher finalizes the directive by saying, "That is correct," followed by the dispatcher's initials. A mandatory directive cannot take effect until the dispatcher has said, "That is correct," reducing the potential for human error to cause an accident.
Numbers are always repeated in an alternate format to ensure clarity and prevent misunderstandings. Numbers from zero to nine are spelled out, while numbers greater than ten are given by their individual numerals. For example, the number six would be given as, "Six, s-i-x," while the number 14 would be given as "fourteen, one-four." Engine numbers such as 2001 are usually read first as "two thousand one," while numbers such as 4321 are usually read as "forty-three twenty-one," although this is mostly a personal preference about what is easiest to say. Regardless, the repeat will give the individual numerals, as in, "Engine twenty-eight oh eight, two, eight, zero, eight."
A sample issue format would be as follows:
When trains must meet en route, one train must be authorized into the siding. For example, train 5432 may be authorized from Anna into Bess Siding:
An opposing train will be authorized in two blocks, Cloy through Bess, using a similar format:
Once train 5432 has fully entered the siding and restored the switch for movement along the main track, they will call the dispatcher and release Anna block, using the following format:
After the 5432 has released its authority in Anna, the dispatcher may issue a new authority to the 4321 authorizing them through Anna block. Since only one authority is valid at any given time, if the 4321 is still in Cloy block, the dispatcher will give the new authority containing Cloy through Anna. Once the 4321 is fully within the Bess block and has cleared Cloy block, the conductor or other crew member will call the dispatcher and release Cloy. The dispatcher will then issue a new authority to the 5432, giving them authority from Bess Siding block through Cloy block. This generates additional radio traffic but ensures that there is no confusion as to which train is allowed to enter a given block at any given time. Some railroads have instituted a practice which allows a train to enter a block contingent upon the passing of the opposing train, although the National Transportation Safety Board
has recommended against this practice. A DTC authority given under this practice will include a phrase such as "This line is not in effect until after the arrival of train 7940."
In the case of two trains traveling the same direction, the dispatcher will only authorize the following train in blocks that the leading train has released. In the case of closely spaced trains, the dispatcher may opt to reduce his or her workload and implement a "radio blocking behind" authority, which lets the two trains talk to each other directly to discuss which blocks the leading train has cleared. Caution must be exercised by the leading train not to release authority on track that the train still occupies, as this could result in a rear-end collision if the leading train slows down or stops or if the train becomes separated and a portion is left standing on the main track.
Entering a block in which the train is not authorized or releasing a block which the train still occupies is considered a main-line violation by the Federal Railroad Administration
and carries stiff penalties for all crew members, which may include time off from work and the suspension or revocation of the engineer's license.
, now the Union Pacific, implemented DTC on several of its lines formerly controlled by train order
s (most of the rest of UP's lines use Centralized Traffic Control
). The BNSF Railway
does not use DTC; it uses mostly CTC with track warrant
s in its non-signalled territory. The Alaska Railroad
changed its Track Warrant Control territory to DTC and is also implementing CTC in high-traffic areas. Queensland Rail
in Australia uses the DTC system in some of the lower traffic areas in the western area of the state. CSX uses DTC on their former Georgia railroad mainline from Scottdale, just east of Atlanta, to Augusta.
Railway signal
A signal is a mechanical or electrical device erected beside a railway line to pass information relating to the state of the line ahead to train/engine drivers. The driver interprets the signal's indication and acts accordingly...
. It is known as "direct" traffic control because the train dispatcher gives track authority directly to the train crew via radio, as opposed to through wayside personnel via telephone or telegraph, as in train order
Train order
Train order operation, or more accurately Timetable and Train order operation, is a largely obsolete system by which the railroads of North America conveyed operating instructions before the days of centralized traffic control, direct traffic control, and the use of track warrants conveyed by radio...
s.
Layout
Train dispatcher
A train dispatcher is employed by a railroad to direct and facilitate the movement of trains over an assigned territory, which is usually part, or all, of a railroad operating division. In Canada the train dispatcher is known as the rail traffic controller...
to occupy) are divided into pre-specified blocks. In addition to being listed by milepost in the railroad's timetable
Public transport timetable
A public transport timetable is a representation of public transport information to assist a passenger with planning a trip using public transport. A timetable details when vehicle will arrive and depart specified locations and may be organised for by route or for a particular stop...
, block limits are delineated by conspicuous signs along the tracks. Every portion of controlled track belongs to a block, as blocks are laid out back-to-back along the entire length of the rail line.
For example, a 30 miles (48.3 km) length of main line track may be divided into three blocks, Anna, Bess, and Cloy, each 10 miles (16.1 km) long. At milepost 10, there will be a sign displaying the end of Anna block and the beginning of Bess block, and a similar sign between Bess and Cloy. A train authorized in Anna through Bess blocks must stop before Bess block ends and Cloy block begins.
Lengths of DTC blocks vary but usually take about 10 minutes to traverse. However, where the main track is paralleled by a controlled siding
Rail siding
A siding, in rail terminology, is a low-speed track section distinct from a running line or through route such as a main line or branch line or spur. It may connect to through track or to other sidings at either end...
, the main track block is the length of the siding. The siding is a considered a separate block. In DTC, most sidings are considered controlled track and must not be occupied without authority from the train dispatcher. Sidings are considered blocks in the same way as the main track; in fact, from a technical standpoint, very little differentiates a siding from a main track in DTC.
For example, a length of track may consist of three blocks. Anna, Bess, and Cloy. There is a siding at Bess, and this siding is specified as Bess Siding. Both Bess and Bess Siding blocks extend to the switches at either end of the siding. Anna and Cloy blocks begin on the other side of their respective switches. Neither Bess nor Bess Siding blocks may be occupied without authority from the train dispatcher.
In this example, a train may have authority straight through from Anna through Cloy. Or, it could have authority in Anna through Bess Siding, in which case it must throw the switch between Anna and Bess and enter the siding--this will usually be done to clear up the main track in preparation for an opposing train to pass. Then, it might get authority from Bess Siding through Cloy, in which case it will throw the switch between Bess and Cloy and reenter the main track.
While most DTC blocks are laid out back-to-back, the blocks actually do not touch at sidings. Instead, there is a "gray area" over the siding's switches, and the main track's and siding's DTC blocks begins at approximately the clearance point of the switch. The area over the switch must not be occupied unless authority is granted in blocks on both sides of the switch. For example, a train authorized only in Anna block must stop at the End Anna Block sign, which is a short distance from the switch points. A train authorized in Anna through Bess or in Anna through Bess Siding may proceed past the End Anna Block sign, over the switch, and then past the Begin Bess (or Bess Siding, if appropriate) Block sign.
Blocks are typically named after a distinguishing feature within the block, such as a station, town, or river.
Procedures
Normally, only one train or piece of equipment may occupy a given block at a time, and a train must release their authority in a block in order for the train dispatcher to authorize another train into that block. Barring human error, this ensures that no two trains are ever authorized on any given piece of track at the same time, thereby preventing collisions. (In certain circumstances, dispatchers will authorize more than one piece of equipment--as in the case of track maintenance vehicles--into the same block. When this occurs, the authority is marked as "joint," and all movements must be made at restricted speed, which allows stopping within half the range of vision.) Another key feature of DTC is that only one authority is in effect for any given train at any given time. When a dispatcher issues a new authority to a train, the previous authority becomes invalid and must not be used.In modern implementations, dispatchers rely on computerized systems to keep track of trains that have received authority. Typically, the computer will prevent the dispatcher from giving two trains authority over the same track. The computer system generally displays a highly simplified mock-up of the track, displaying the block limits and sidings. Track occupancy is displayed via bold or colored lines overlaying the track display, along with tags to identify the train (usually the number of the lead locomotive).
To receive or change a train's authority, the train dispatcher communicate by radio with the train crew and gives "mandatory directives" authorizing occupancy in a specified number of blocks. A member of the train crew, usually the conductor, copies the dispatcher's mandatory directive onto a prescribed form. The crew member will repeat the directive back to the dispatcher, who monitors the repeat for any errors. If the crew member correctly repeats the mandatory directive, the dispatcher finalizes the directive by saying, "That is correct," followed by the dispatcher's initials. A mandatory directive cannot take effect until the dispatcher has said, "That is correct," reducing the potential for human error to cause an accident.
Numbers are always repeated in an alternate format to ensure clarity and prevent misunderstandings. Numbers from zero to nine are spelled out, while numbers greater than ten are given by their individual numerals. For example, the number six would be given as, "Six, s-i-x," while the number 14 would be given as "fourteen, one-four." Engine numbers such as 2001 are usually read first as "two thousand one," while numbers such as 4321 are usually read as "forty-three twenty-one," although this is mostly a personal preference about what is easiest to say. Regardless, the repeat will give the individual numerals, as in, "Engine twenty-eight oh eight, two, eight, zero, eight."
A sample issue format would be as follows:
When trains must meet en route, one train must be authorized into the siding. For example, train 5432 may be authorized from Anna into Bess Siding:
An opposing train will be authorized in two blocks, Cloy through Bess, using a similar format:
Once train 5432 has fully entered the siding and restored the switch for movement along the main track, they will call the dispatcher and release Anna block, using the following format:
After the 5432 has released its authority in Anna, the dispatcher may issue a new authority to the 4321 authorizing them through Anna block. Since only one authority is valid at any given time, if the 4321 is still in Cloy block, the dispatcher will give the new authority containing Cloy through Anna. Once the 4321 is fully within the Bess block and has cleared Cloy block, the conductor or other crew member will call the dispatcher and release Cloy. The dispatcher will then issue a new authority to the 5432, giving them authority from Bess Siding block through Cloy block. This generates additional radio traffic but ensures that there is no confusion as to which train is allowed to enter a given block at any given time. Some railroads have instituted a practice which allows a train to enter a block contingent upon the passing of the opposing train, although the National Transportation Safety Board
National Transportation Safety Board
The National Transportation Safety Board is an independent U.S. government investigative agency responsible for civil transportation accident investigation. In this role, the NTSB investigates and reports on aviation accidents and incidents, certain types of highway crashes, ship and marine...
has recommended against this practice. A DTC authority given under this practice will include a phrase such as "This line is not in effect until after the arrival of train 7940."
In the case of two trains traveling the same direction, the dispatcher will only authorize the following train in blocks that the leading train has released. In the case of closely spaced trains, the dispatcher may opt to reduce his or her workload and implement a "radio blocking behind" authority, which lets the two trains talk to each other directly to discuss which blocks the leading train has cleared. Caution must be exercised by the leading train not to release authority on track that the train still occupies, as this could result in a rear-end collision if the leading train slows down or stops or if the train becomes separated and a portion is left standing on the main track.
Entering a block in which the train is not authorized or releasing a block which the train still occupies is considered a main-line violation by the Federal Railroad Administration
Federal Railroad Administration
The Federal Railroad Administration is an agency in the United States Department of Transportation. The agency was created by the Department of Transportation Act of 1966...
and carries stiff penalties for all crew members, which may include time off from work and the suspension or revocation of the engineer's license.
Use
The Southern Pacific RailroadSouthern Pacific Railroad
The Southern Pacific Transportation Company , earlier Southern Pacific Railroad and Southern Pacific Company, and usually simply called the Southern Pacific or Espee, was an American railroad....
, now the Union Pacific, implemented DTC on several of its lines formerly controlled by train order
Train order
Train order operation, or more accurately Timetable and Train order operation, is a largely obsolete system by which the railroads of North America conveyed operating instructions before the days of centralized traffic control, direct traffic control, and the use of track warrants conveyed by radio...
s (most of the rest of UP's lines use Centralized Traffic Control
Centralized traffic control
Centralized traffic control is a form of railway signalling that originated in North America and centralizes train routing decisions that were previously carried out by local signal operators or the train crews themselves. The system consists of a centralized train dispatcher's office that...
). The BNSF Railway
BNSF Railway
The BNSF Railway is a wholly owned subsidiary of Berkshire Hathaway Inc., and is headquartered in Fort Worth, Texas. It is one of seven North American Class I railroads and the second largest freight railroad network in North America, second only to the Union Pacific Railroad, its primary...
does not use DTC; it uses mostly CTC with track warrant
Track warrant
Track warrants are systematized permissions used on some railroad lines to authorize a train's use of the main line. Dispatchers issue these permissions to train crews instead of using signals. The crews receive track warrants by radio, phone, or electronic transmission from a...
s in its non-signalled territory. The Alaska Railroad
Alaska Railroad
The Alaska Railroad is a Class II railroad which extends from Seward and Whittier, in the south of the state of Alaska, in the United States, to Fairbanks , and beyond to Eielson Air Force Base and Fort Wainwright in the interior of that state...
changed its Track Warrant Control territory to DTC and is also implementing CTC in high-traffic areas. Queensland Rail
Queensland Rail
Queensland Rail, also known as QR, is a government-owned railway operator in the state of Queensland. Under the control of the Queensland Government, Queensland Rail operates the inner-city and long-distance passenger services, as well as some freight operations and gives railway access to other...
in Australia uses the DTC system in some of the lower traffic areas in the western area of the state. CSX uses DTC on their former Georgia railroad mainline from Scottdale, just east of Atlanta, to Augusta.
See also
- Rail terminologyRail terminologyRail terminology is a form of technical terminology. The difference between the American term railroad and the international term railway is the most obvious difference in rail terminology...
- Railway signallingRailway signallingRailway signalling is a system used to control railway traffic safely, essentially to prevent trains from colliding. Being guided by fixed rails, trains are uniquely susceptible to collision; furthermore, trains cannot stop quickly, and frequently operate at speeds that do not enable them to stop...
- Train orderTrain orderTrain order operation, or more accurately Timetable and Train order operation, is a largely obsolete system by which the railroads of North America conveyed operating instructions before the days of centralized traffic control, direct traffic control, and the use of track warrants conveyed by radio...
- Track warrantTrack warrantTrack warrants are systematized permissions used on some railroad lines to authorize a train's use of the main line. Dispatchers issue these permissions to train crews instead of using signals. The crews receive track warrants by radio, phone, or electronic transmission from a...
- Centralized Traffic ControlCentralized traffic controlCentralized traffic control is a form of railway signalling that originated in North America and centralizes train routing decisions that were previously carried out by local signal operators or the train crews themselves. The system consists of a centralized train dispatcher's office that...